After completing this chapter, the reader should be able to
Describe the role of platelets, the coagulation cascade, and fibrinolytic system in normal hemostasis
List the laboratory tests used to assess platelets and discuss factors that may influence their results
List the laboratory tests used to assess coagulation and explain their use in evaluating anticoagulant therapy
List the laboratory tests used to assess clot degradation and disseminated intravascular coagulation and discuss their limitations
Interpret results and suggest follow-up action given results of laboratory tests used for evaluating coagulation and anticoagulant therapy in a case description
Discuss the availability and use of point-of-care testing devices specifically for platelet and coagulation tests
Normal hemostasis involves a complex interaction among the vascular subendothelium, platelets, coagulation factors, and proteins that promote clot formation, clot degradation, and inhibitors of these substances. Disruption in normal hemostasis can result in bleeding or excessive clotting. Bleeding can be caused by trauma or damage to vessels, acquired or inherited deficiencies of coagulation factors, or physiologic disorders of platelets, whereas excessive clotting can result from abnormalities of the vascular endothelium, alterations in blood flow, or deficiencies in clotting inhibitors.
Clinicians must monitor the hemostasis process in individual patients to ensure their safety from an imbalance in this complex system. For example, practitioners routinely order platelet tests in patients on certain antineoplastic medications to assess for thrombocytopenia. Likewise, clinicians may closely monitor coagulation tests for patients receiving certain anticoagulants to prevent thromboembolic or hemorrhagic complications; however, it should be noted that not all anticoagulants are routinely monitored, nor are all laboratory tests available for routine clinical use. Overall, the hemostatic process is intricate and requires a clinician knowledgeable in its dynamics for quality assessment.
This chapter reviews normal coagulation physiology, common tests used to assess coagulation and hypercoagulable states, and factors that alter coagulation tests.
PHYSIOLOGIC PROCESS OF HEMOSTASIS
Normal hemostasis involves the complex relationship among participants that promotes clot formation (platelets and the coagulation cascade), inhibits coagulation, and dissolves the formed clot. Each phase of the process is briefly reviewed.
Numerous mechanisms promote and limit coagulation. Factors that promote coagulation include malignancy, pregnancy, obesity, immobilization, damage to the blood vessel wall, and causes of low blood flow or venous stasis. Certain medications may also increase risk of thrombosis, including estrogen, tamoxifen, thalidomide, and erythropoietin.1 Normal blood flow dilutes activated clotting factors and results in their degradation in various tissues (eg, liver) and by proteases. However, when low flow or venous stasis is present, activated clotting factors may not be readily cleared.
Platelets are nonnucleated, disk-shaped structures, 1 to 5 μm in diameter, that are formed in the extravascular spaces of bone marrow from megakaryocytes. Megakaryocyte production and maturation are promoted by the hormone thrombopoietin, which is synthesized in the bone marrow and liver. Two-thirds of the platelets are found in the circulation and one-third is found in the spleen; however, in splenectomized patients, nearly 100% is in the circulation.
The average human adult makes approximately 100 billion platelets per day, with the average platelet circulating for 7 to 10 days. On aging, platelets are destroyed by the spleen, liver, and bone marrow. Throughout their lifespan, platelet function is affected by numerous factors, such as medications, vitamins, foods, spices, and systemic conditions, including chronic renal disease and hematologic disorders (eg, myeloproliferative and lymphoproliferative diseases, dysproteinemias, and the presence of antiplatelet antibodies).
The primary function of platelets is to regulate hemostasis, but platelets also play a prominent role in the pathologic formation of arterial thrombi. Three processes (platelet adhesion, activation, and aggregation) are essential for arterial thrombus formation. The surface of normal blood vessels inhibits platelet function, thereby preventing thrombosis; however, endothelial injury to the vasculature, caused by flow abnormalities, trauma, or the rupture of atherosclerotic plaque in the vessel wall, starts the process of platelet plug formation. Subendothelial structures, such as collagen, then become exposed (Figure 17-1), which can result in platelet adhesion mediated by von Willebrand factor (vWF); platelet adhesion is enhanced by substances such as epinephrine, thrombin, adenosine diphosphate (ADP), and serotonin.2 Circulating vWF acts as a binding ligand between the subendothelium and glycoprotein Ib receptors on the platelet surface.
Once adhesion occurs, platelets change shape and activation occurs. Activated platelets release their contents—including nucleotides, adhesive proteins, growth factors, and procoagulants—which promotes platelet aggregation and completes the formation of the hemostatic plug.3 This process is mediated by glycoprotein IIb/IIIa receptors on the platelet surface, with fibrinogen acting as the primary binding ligand bridging between platelets. Platelets have numerous glycoprotein IIb/IIIa binding sites, which are an attractive option for antiplatelet drug therapy.2 However, the platelet plug is not stable and can be dislodged. To form a more permanent hemostatic plug, the clotting system must be stimulated. By releasing platelet factor (PF) 3, platelets initiate the clotting cascade and concentrate activated clotting factors at the site of vascular (endothelial) injury.
Prostaglandins (PGs) play an important role in platelet function. Figure 17-2 displays a simplified version of the complex arachidonic acid pathways that occur in platelets and on the vascular endothelium. Thromboxane A2, a potent stimulator of platelet aggregation and vasoconstriction, is formed in platelets. In contrast, prostacyclin (PGI2), produced by endothelial cells lining the vessel luminal surface, is a potent inhibitor of platelet aggregation and a potent vasodilator that limits excessive platelet aggregation.
Cyclooxygenase and PGI2 are clinically important. An aspirin dose of 50 to 81 mg/day acetylates and irreversibly inhibits cyclooxygenase in the platelet. Platelets are rendered incapable of converting arachidonic acid to PGs. This effect of low-dose aspirin lasts for the lifespan of the exposed platelets (8 to 12 days). Vascular endothelial cells also contain cyclooxygenase, which converts arachidonic acid to PGI2. Aspirin in high doses (3,000 to 5,000 mg) inhibits the production of PG2.4 However, because the vascular endothelium can regenerate PGI2, aspirin’s effect is much shorter here than on platelets. Thus, aspirin’s effect at high doses may both inhibit platelet aggregation and block the aggregation inhibitor PGI2. This phenomenon is the rationale for using low doses of aspirin (75 to 162 mg/day) to help prevent MI.
In summary, a complex interaction between the platelet and blood vessel wall maintains hemostasis. Once platelet adhesion occurs, the clotting cascade may become activated. After thrombin and fibrin are generated, the platelet plug becomes stabilized with insoluble fibrin at the site of vascular injury.
The ultimate goal of the coagulation cascade (Figure 17-3) is to generate fibrin from thrombin. Fibrin forms an insoluble mesh surrounding the platelet plug. Platelets concentrate activated clotting factors at the site of vascular injury.
In addition to the direct effects and feedback mechanisms of thrombin shown in Figure 17-3, thrombin also stimulates platelet aggregation and activates the fibrinolytic system.
Additional factors within the pathway
Factors such as calcium and vitamin K play an intricate role within the various pathways in the coagulation cascade. Calcium is essential for the platelet surface binding of several factors within the pathway. Vitamin K facilitates the calcium binding function of factors II, VII, IX, and X via carboxylation. These processes are critical in activating proteins within the pathway.
Inhibition of Coagulation
Mechanisms that limit coagulation include the natural inhibitors such as antithrombin (AT) and the vitamin K-dependent proteins C and S, tissue factor pathway inhibitor (TFPI), and the fibrinolytic system. Endothelial cells produce several substances that have antithrombotic and anticoagulant effects, which may also activate the fibrinolytic system.2 Several medications also can inhibit coagulation by acting on (1) platelets, such as aspirin and P2Y12 inhibitors or (2) one or more clotting factors, such as warfarin; low molecular weight heparins (LMWHs); unfractionated heparin (UFH); direct oral anticoagulants (DOACs); fondaparinux; and direct thrombin inhibitors (DTIs). Table 17-1 lists the mechanism of action of these classes of drugs and provides specific examples.
Mechanism of Action of Antithrombotic and Anticoagulant Medications
MECHANISM OF ACTION
Irreversibly inhibits cyclooxygenase-1, which prevents conversion of arachidonic acid to thromboxane A2
Irreversibly binds to P2Y12 receptors on platelets
Warfarin (Coumadin, Jantoven)
Inhibits vitamin K-dependent clotting factors (II, VII, IX, X) as well as protein C and protein S
Inhibits factor Xa
Inhibits factor IIa
Inhibits factors IIa and Xa
Inhibits factor Xa
High concentrations of thrombin, in conjunction with thrombomodulin, activate protein C, which can then inactivate cofactors Va and VIIIa. Thus, there is a negative feedback mechanism that blocks further thrombin generation and subsequent steps in the coagulation cascade. Protein S is another of the body’s natural anticoagulants and serves as a cofactor for protein C. AT inactivates thrombin as well as factors IX, X, and XI, and this process can be hastened by heparin. Heparin and AT combine one-to-one, and the complex neutralizes the activated clotting factors and inhibits the coagulation cascade. Deficiencies in these natural inhibitors can result in increased generation of thrombin, which can lead to recurrent thromboembolic events often starting at a young age; the prevalence of protein C or protein S deficiency in the general population is estimated at <0.5%.5,6 Patients with either protein C or protein S deficiency are at a higher risk of warfarin-induced skin necrosis compared with individuals without these deficiencies.7–10 TFPI impedes the binding of tissue factor (TF) to factor VII, essentially inhibiting the extrinsic pathway (Figure 17-3). UFH and LMWHs can release TFPI from endothelial cells and from platelets.2 The complex mechanisms that limit thrombus formation are shown in Figure 17-4.
Fibrinolysis is the mechanism by which formed thrombi are lysed to prevent excessive clot formation and vascular occlusion. As discussed previously, fibrin is formed in the final common pathway of the clotting cascade. Tissue plasminogen activator and urokinase plasminogen activator activate plasminogen, which generates plasmin. Plasmin is the enzyme that eventually breaks down fibrin into fibrin degradation products (FDPs). Medications can either activate (eg, alteplase, reteplase, and tenecteplase) or inhibit (eg, tranexamic acid and aminocaproic acid) fibrinolysis.
TESTS TO EVALUATE HEMOSTASIS
For the purpose of discussion, bleeding and clotting disorders are organized by tests that assess platelets, coagulation, and clot degradation. Tests to assess platelets include platelet count, volume (eg, mean platelet volume [MPV]), function (eg, bleeding time [BT] and platelet aggregation), and others. Prothrombin time (PT)/International Normalized Ratio (INR), activated partial thromboplastin time (aPTT), activated clotting time (ACT), fibrinogen assay, thrombin time (TT), and others are laboratory tests that assess coagulation; a hypercoagulable panel can be drawn to determine whether a patient has one or more hypercoagulable disorders. Clot degradation is assessed with tests for FDPs and d-dimer.
In addition, general hematologic values such as hemoglobin (Hgb), hematocrit (Hct), red blood cell (RBC) count, and white blood cell count, as well as urinalysis and stool guaiac tests may be important to obtain when evaluating blood and coagulation disorders; some of these tests are further discussed in Chapter 16. Table 17-2 is a summary of common tests used to evaluate bleeding disorders and monitor anticoagulant therapy.
Summary of Coagulation Tests for Hemorrhagic Disorders and Anticoagulant Drug Monitoring
DISORDER OR DRUG
Platelets appear normal
von Willebrand disease
Low or WNL
WNL or prolonged
Factor VIII levels low or WNL, vWF (antigen level and activity) low or WNL
Fibrinogen levels decreased
WNL or prolonged
WNL or prolonged
WNL or prolonged (LMWH)
Platelet count may decrease
WNL to prolonged
Direct Xa inhibitors
WNL to prolonged
Normal platelet count distinguishes this from other forms of purpura, such as TTP or ITP
ITP= idiopathic thrombocytopenic purpura; TTP= thrombotic thrombocytopenic purpura; WNL= within normal limits.
Normal range: 150,000 to 450,000/mL (150 to 450 × 109/L)
The only test to determine the number or concentration of platelets in a blood sample is the platelet count, through either manual (rarely done) or automated methods. Interferences with platelet counts include RBC fragments, platelet clumping, and platelet satellitism (platelet adherence to white blood cells). Automated platelet counts are performed on anticoagulated whole blood. Most instrumentation that performs hematologic profiles provides platelet counts. Platelets and RBCs are passed through an aperture, generating an electric pulse with a magnitude related to the size of the cell/particle. The pulses are counted, and the platelets are separated from the RBCs by size, providing the platelet count and MPV as well as the RBC count and mean corpuscular volume. An abnormal platelet count can have many causes, which are listed in the next sections.
Thrombocythemia, also known as thrombocytosis or elevated platelet count, may be considered either primary or secondary. Primary thrombocytosis is usually caused by myeloproliferative neoplasms and other hematologic malignancies.11 Secondary or reactive thrombocytosis may be caused by factors such as:3,11
Values of 500,000 to 800,000/μL are not uncommon. Thrombocythemia may be seen with any of the chronic myeloproliferative neoplasms, essential thrombocythemia, polycythemia vera, chronic myelogenous leukemia, or idiopathic myelofibrosis. Clinical consequences of thrombocythemia include arterial or venous thromboses, skin and mucous membrane hemorrhages, and microcirculatory disturbances such as headaches, paresthesias, and erythromelalgia.11 Additionally, patients with thrombocythemia may have abnormalities in platelet function studies, which can manifest as bleeding problems.
The main causes of thrombocytopenia, or decreased platelet count, are (1) increased destruction or consumption of platelets, (2) decreased production, and (3) sequestration.3 Mucosal and cutaneous bleeding is the most common clinical consequence of thrombocytopenia; however, patients with only modest decreases in platelet counts may be asymptomatic. When the platelet count falls below 20,000/μL, the patient is at risk for spontaneous bleeding. Platelet transfusions are often initiated when counts fall below 10,000/μL, but this number can vary based on individual patient risk factors and clinical situation.12 Bleeding may occur at higher platelet counts (eg, 50,000/μL) if trauma occurs. The most common cause of death in a patient with severe thrombocytopenia is central nervous system bleeding, such as intracranial hemorrhage.
Numerous drugs have been associated with thrombocytopenia (Table 17-3).13,14 However, heparin and antineoplastics are the most common ones implicated. Thrombocytopenia is also common with radiation therapy. Many drugs associated with thrombocytopenia alter platelet antigens, resulting in the formation of antibodies to platelets (eg, heparin, penicillin, and gold). Several diseases, such as thrombotic thrombocytopenic purpura, idiopathic thrombocytopenic purpura, disseminated intravascular coagulation (DIC), and hemolytic-uremic syndrome, result in rapid destruction of platelets. Other causes of thrombocytopenia include viral infections; pernicious, aplastic, and folate or B12-deficiency anemias; complications of pregnancy; massive blood transfusions; exposure to dichlorodiphenyltrichloroethane; and human immunodeficiency virus infections.
Partial List of Agents Associated with Thrombocytopenia
Source: Adapted with permission from references 13 and 14.
Heparin-induced thrombocytopenia (HIT) is an antibody-mediated adverse reaction to heparin, occurring in 1 in 5,000 hospitalized patients, that may cause venous and arterial thrombosis.15 Specifically, this is due to the development of immunoglobulin G antibodies that bind to the heparin PF4 complex. Patients receiving UFH generally have 10-fold greater risk of developing HIT than patients receiving LMWH because it does not bind to PF4 as well as UFH, which is thought to be due to the smaller size of LMWH compared with UFH.16 Therefore, the heparin-PF4 complex is less likely to form with LMWH, and there are fewer immunoglobulin G antibodies generated. The frequency or risk of HIT is influenced by certain factors, such as heparin preparation, route, dose, and duration of heparin therapy, patient population, gender, previous history of heparin exposure, and the animal source of heparin (bovine versus porcine).17
The 4Ts score is a clinical prediction tool to determine the probability of HIT. This tool requires the clinician to evaluate the degree of thrombocytopenia, timing of platelet count fall, presence of thrombosis or other clinical sequelae, and other causes for thrombocytopenia; a score of 0 to 2 is assigned for each of the four items based on specific patient characteristics to determine the probability of HIT occurring in that particular patient. Thus, the score range is 0 to 8. A score of 0 to 3, 4 to 5, or 6 to 8 suggests a low, moderate, or high probability of HIT, respectively.18 HIT is manifested both by clinical and serological features, and diagnosis of HIT is usually made when antibody formation is detected by an in vitro assay plus one or more of the following: unexplained decrease in platelet count, venous or arterial thrombosis, limb gangrene, necrotizing skin lesions at the heparin injection site, acute anaphylactoid reactions occurring after intravenous (IV) heparin bolus administration, and/or bleeding.17,19
There are two types of tests to help diagnose HIT: (1) the enzyme-linked immunosorbent assay (ELISA), which identifies anti-PF4/heparin antibodies, and (2) functional assays, such as the C-serotonin release assay or the heparin-induced platelet activation assay—both of which detect antibodies that induce heparin-dependent platelet activation.15,20 The ELISA test has high sensitivity, low specificity, and wide availability, with a relatively rapid turnaround time compared with the functional assays, which makes it a good screening test.18 By contrast, the functional assays have high specificity, which are useful for confirming a positive ELISA test but are technically difficult, have a higher cost, and have longer turnaround time.18
The typical onset for HIT is 5 to 10 days after the start of heparin; however, onsets occurring either earlier or later than this have been reported. Rapid-onset HIT occurs when platelet counts fall within 24 hours of heparin initiation, which is typically the result of repeated heparin exposure within the past 100 days, and thus patients still have circulating HIT antibodies. Delayed-onset HIT, in which thrombocytopenia occurs several days after discontinuation of heparin, can also occur. Table 17-4 outlines the patient characteristics associated with the risk of developing HIT as well as recommendations for monitoring platelet counts.20,21
Incidence of HIT According to Patient Characteristics and Recommendations for Monitoring Platelets
RISK OF DEVELOPING HIT
0.1% TO 1%
Postoperative patients on prophylactic dose or therapeutic dose UFH ≥4 days
Medical patients on prophylactic or therapeutic-dose UFH or LMWH ≥4 days
Cardiac surgery patients
Postoperative patients on prophylactic or therapeutic dose LMWH ≥4 days
Patients receiving UFH flushes
Intensive care patients
Surgical and trauma patients receiving UFH
Medical and obstetrical patients receiving UFH
Medical and obstetrical patients receiving LMWH
Patients receiving LMWH after major surgery or major trauma
Patients receiving LMWH after minor surgery or trauma
Patients receiving fondaparinux
Frequency of platelet counts
Every 2–3 days from days 4–14, or until heparin is discontinued, whichever occurs first
Routine monitoring is not recommended
Routine monitoring is not recommended
If patient has received heparin in the last 30 days prior to current regimen, begin monitoring on day 0, when heparin is initiated
If patient has received heparin in the last 30 days prior to current regimen, begin monitoring on day 0, when heparin is initiated
If patient has not received heparin in the last 30 days prior to current regimen, monitor at least every other day from days 4–14, or until heparin is discontinued, whichever occurs first
If patient has not received heparin in the last 30 days prior to current regimen, monitor every 2–3 days from days 4–14, or until heparin is discontinued, whichever occurs first
ASH = American Society of Hematology
Source: Adapted with permission references 20 and 21.
Mean Platelet Volume
Normal range: 7 to 11 fL (varies with laboratory)
Mean platelet volume (MPV)—the relationship between platelet size and count—is most likely used by clinicians in assessing disturbances of platelet production. MPV is useful in distinguishing between hypoproductive and hyperdestructive causes of thrombocytopenia (Figure 17-5). Despite the widespread availability of this platelet index, many clinicians do not use it in clinical decision-making. In the past, this disuse was attributed to difficulties with the laboratory measurement of indices.
Many laboratories routinely report the MPV as part of the complete blood count, especially if a differential is requested. In general, lower platelet counts are common with higher platelet volumes because an inverse relationship exists between the platelet count and the MPV. This inverse relationship correlates with platelet production within the bone marrow. Although MPV is most valuable in distinguishing hypoproductive from hyperdestructive causes of thrombocytopenia, a definitive diagnosis cannot be made based on MPV alone. In thrombocytopenia, an elevated MPV suggests no problem with platelet production, when in fact, production is reflexively increased. Conversely, a normal or low MPV suggests impaired thrombopoiesis. Determination of MPV requires a blood collection tube containing an anticoagulant. Usually, such tubes contain the anticoagulant ethylenediamine tetraacetic acid (EDTA), which causes an inflation of the MPV.22Table 17-5 lists conditions that affect the MPV.23,24
Conditions Associated with Alterations in MPV
INCREASE IN MPV
DECREASE IN MPV
Nonalcoholic fatty liver disease
Systemic lupus erythematosus
Decompensated heart failure
Pulmonary arterial hypertension
Chronic obstructive pulmonary disease
Obstructive sleep apnea
HIV = human immunodeficiency virus.
Source: Adapted with permission from references 23 and 24.
Simply stated, the platelet function tests look at the ability of platelets to aggregate and form a clot. Abnormalities of platelet function may be either inherited or acquired and can be caused by medications, the platelet milieu, and inherent platelet defects; platelet counts are usually normal.25 Bleeding as a result of an inherited versus acquired abnormality may be difficult to prove. Common bleeding sites in patients with disorders of platelet function include ecchymosis of the skin, epistaxis, gingival bleeding, and menorrhagia; gastrointestinal (GI) hemorrhage and hematuria are less common and usually have an associated underlying pathology.
Although the sites of bleeding may be predictable, the severity is not predictable in patients with inherited disorders of platelet function. Unfortunately, the risk of bleeding and bleeding patterns in patients with acquired platelet dysfunction are less predictable and more difficult to distinguish. Because both inherited and acquired etiologies increase the risk of bleeding, patients overtly bleeding without a clear cause or without an invasive procedure should be evaluated for one of these platelet function disorders.
Normal range: 2 to 9 minutes
Bleeding time (BT) is a measure of platelet function and has been used to assess bleeding risk, but this test is neither specific nor sensitive; thus, it does not help differentiate among the types of problems seen in disorders of primary hemostasis, such as von Willebrand disease and platelet function defects. Because BT is neither specific nor sensitive, its use has been declining, and some institutional clinical laboratories no longer perform this test. Additionally, the test is invasive and must be performed by a trained healthcare worker. To perform the test, small cuts are made on the forearm of the patient, and the time it takes to stop bleeding is measured. Several factors can prolong the BT, including thrombocytopenia, certain medications, and conditions such as uremia and macroglobulinemia. Most acquired disorders affecting BT are related to medications that decrease platelet numbers or reduce platelet function, including aspirin, P2Y12 inhibitors (clopidogrel, prasugrel, ticagrelor), GPIIb/IIIa inhibitors (abciximab, eptifibatide, tirofiban), and phosphodiesterase inhibitors (dipyridamole). Although BT is influenced by some drugs, it is not used to monitor drug therapy. The increase in BT caused by aspirin may have beneficial effects in the treatment and prevention of cardiovascular disease.
With the many drawbacks of the BT, there was a need for a test that could aid in the diagnosis of defects in platelet function. The ability of platelets to aggregate is most commonly measured by preparing a specimen of platelet-rich plasma and warming it to 98.6°F (37°C) with constant stirring. This test is performed with an aggregometer that measures light transmission through a sample of platelets in suspension. After a baseline reading is obtained, a platelet-aggregating agonist (eg, epinephrine, collagen, ADP, or arachidonic acid) is added. As platelets aggregate, more light passes through the sample. The change in optical density can be measured photometrically and recorded as an aggregation curve, which is then printed on a plotter. Although light transmittance aggregometry (LTA) testing is the gold standard in platelet function analysis, it has requirements for specially trained personnel, large sample volume, and sample preparation, and it is costly.22
Interpretation of platelet aggregation tests involves a comparison of the patient’s curves with the corresponding curves of a normal control. To eliminate the optical problems of turbidity with lipemic plasma, the patient and the normal control should be fasting. Patients should not take medications that affect platelet aggregation (eg, aspirin, nonsteroidal anti-inflammatory drugs, P2Y12 inhibitors) for approximately 7 to 14 days before the test because they may interfere with test results. Other drugs that may affect platelet function are listed in Table 17-6.26
Medications and Drug Classes That May Cause Abnormalities of Platelet Function
Source: Adapted with permission from reference 26.
Novel point-of-care (POC) technologies are available and allow for rapid and meaningful evaluation of platelet function, although major differences between devices do exist. These devices can assess the effects of medications such as aspirin, P2Y12 inhibitors, and GP IIb/IIIa antagonists on platelet function and help predict the incidence of major adverse cardiac events in patients treated with medications affecting platelet function.27 Further studies of each individual device are needed to elucidate the exact place in therapy of these in monitoring antiplatelet medications.
Other Platelet Tests
The measurement of platelet-specific substances, such as PF4 (normal values 1.7 to 20.9 ng/mL) and β-thromboglobulin (normal levels 6.6 to 47.9 ng/mL), can now be performed by radioimmunoassay or enzyme immunoassay.28 High concentrations of these substances may be observed with coronary artery disease, acute MI, and thrombosis, in which platelet lifespan is reduced. Because numerous drugs can potentially cause thrombocytopenia, detection of antibodies directed by specific drugs against platelets may help to determine the culprit. Platelet survival can be measured by injecting radioisotopes that label the platelets. Serial samples can then determine platelet survival, which is normally 8 to 12 days.
Pharmacogenomics and clopidogrel metabolism
Genetic variability in the genes coding for CYP2C19 may have an effect on clopidogrel efficacy and safety; this concept is covered in more detail in Chapter 6. There are commercially available assays to test for these variants in CYP2C19.29,30 Guidelines do not recommend routine platelet function testing or genetic testing in all patients taking clopidogrel; instead, they recognize a possible role for testing patients who undergo high-risk percutaneous coronary intervention (PCI) procedures, such as those involving bifurcating left main artery.31–33
Coagulation tests are useful in the identification of deficiencies of coagulation factors responsible for bleeding as well as thrombotic disorders. The most commonly performed tests, including PT, INR, aPTT, and ACT, are used to monitor anticoagulant therapy. Numerous high-precision automated laboratory methods are available to perform these tests. However, an overall lack of standardization across coagulation testing can lead to considerable variation in test results and their interpretation. Normal and therapeutic ranges established for one test method are not necessarily interchangeable with other methods, especially when differences in endpoint detection or reagents exist. Therefore, it is important to interpret test results based on the specific performance characteristics of the method used to analyze samples.
Coagulation studies may be used to assess certain bleeding disorders, such as hemophilia A (factor VIII deficiency) or hemophilia B (factor IX deficiency). These deficiencies, which are inherited, sex-linked recessive traits, primarily affect males and cause >90% of hemophilia cases. Other bleeding disorders include von Willebrand disease—the most common hereditary bleeding disorder—and deficiencies in fibrinogen or factors II, V, VII, X, XI, XIII, and a combination of these factors.
Patients with thrombotic disorders may have their hypercoagulability evaluated with specific assays for the following conditions5,6:
Activated protein C (APC) resistance mutation (factor V Leiden)
These tests are often performed in panels because the presence of more than one predisposition to thrombosis further increases the risk for thrombosis. Normal reference ranges for AT and proteins C and S are often reported as a percent of normal activity, with 100% being the mean normal value. For AT, the normal activity level is 80% to 130%; for both proteins C and S, normal activity levels are 70% to 140%. Deficiencies can result in frequent, recurrent thromboembolic events in patients with these disorders. Because these deficiencies are rare, their respective assays are not discussed here in detail. The use for thrombophilia testing is controversial with no clear guidelines on which patients should be tested.5,6,34 Negative results may falsely reassure patients and/or clinicians that the risk of recurrent venous thromboembolism (VTE) is low, leading to discontinuation of anticoagulants, which may put the patient at risk for thrombosis; positive results may lead to continued anticoagulation due to overestimation of the risk of recurrent VTE with the specific thrombophilia, which can put the patient at risk for bleeding.34 Acquired, transient deficiencies of any of these inhibitors may be observed during thrombotic states. Therefore, these parameters should not be assessed during the acute phase of thrombosis or while the patient is currently on anticoagulant therapy because a false-positive result may occur. It is recommended to test for AT, protein C, protein S, and APC resistance after the thrombosis has been resolved when the patient is off heparin or warfarin for a few weeks or off DOACs for at least five half-lives6; the test for prothrombin G20210A mutation and factor V Leiden is not affected by current anticoagulant therapy6 (Minicase 1). The results of thrombophilia testing should be used along with other risk factors for recurrent VTE to help determine whether a patient should continue anticoagulants.
Risk of Recurrent Venous Thromboembolism
Juan R., a 55-year-old man, presents to the anticoagulation clinic to discuss the possible continued need for anticoagulation. He recently completed 3 months of warfarin therapy for a first event deep vein thrombosis. He reports that he has started a new job that does not allow him to readily come in for INR appointments, so he is asking about converting to a DOAC if he needs to stay on anticoagulant medication. He weighs 85 kg and is 72″ tall with reduced renal function. Laboratory results 4 weeks after stopping warfarin are as follows:
Estimated glomerular filtration rate
QUESTION: How should his lab results be interpreted for his risk of recurrent VTE? Should any additional lab work be performed? What are his options for future anticoagulation if needed?
DISCUSSION: The d-dimer is elevated, which means the patient is at higher risk of recurrent VTE and an extended duration of anticoagulation can be considered. The patient and clinician may elect to do a hypercoagulable panel to see if he has any of the conditions for thrombophilia. Because he has been off warfarin for 4 weeks, there is less likelihood of a false-positive result for tests such as protein C deficiency, protein S deficiency, AT, or APC resistance. This patient can be started on a DOAC or warfarin if extended duration of anticoagulation is selected. The patient would need to be counseled on the benefits and drawbacks of DOACs, including fewer dietary issues, fewer drug interactions, higher cost, and need for adherence to therapy due to short half-lives of these agents. If he decides to stay with warfarin, he may be able to do patient self-monitoring or PST after the anticoagulation clinic assesses his ability to perform such care.
Activated protein C resistance due to the factor V Leiden mutation is the most prevalent hereditary predisposition to venous thrombosis. It is present in approximately 5% of the general white population and is less common or rare in other ethnic groups.6 Prothrombin G20210A mutation is the second most common hereditary predisposition to venous thrombosis. DNA-based methods, such as polymerase chain reaction–based assay, are used to determine the presence or absence of a specific mutation at nucleoside position 20210 in the prothrombin gene. A normal test result would show absence of the G20210A mutation.
Although routine laboratory monitoring is not indicated with DOACs, there are some clinical instances when laboratory assessment could be considered, including a thrombotic or hemorrhagic event, perioperative management, suspicion of overdosage/toxicity, renal/hepatic dysfunction, extremes of body weight, trauma, questionable adherence to therapy, concomitant administration with significant drug interactions, advanced age, and after attempted reversal of anticoagulation.35 Although therapeutic concentrations of the actual drug associated with optimal outcomes have not been established for the DOACs, certain coagulation tests are better suited to assess qualitative (presence or absence of drug) versus quantitative (estimates of drug levels) information for specific DOACs, which are discussed in detail later and summarized in Table 17-7.
Summary of Interpreting Laboratory Tests with DOACs
TESTS THAT MAY BE USED FOR QUALITATIVE ASSESSMENT OF DOACs
TESTS THAT MAY BE USED FOR QUANTITATIVE ASSESSMENT OF DOACs
TESTS NOT RECOMMENDED
aPTT: normal to prolonged
PT: normal to prolonged
Anti-Xa: no effect
Factor Xa inhibitors
(apixaban, edoxaban, rivaroxaban, betrixaban)
PT: normal to prolonged (recommended for rivaroxaban only)
Anti-Xa (if not calibrated to a specific DOAC or if calibrated to UFH or LMWH)
Anti-Xa (only if calibrated to a specific DOAC)
aPTT: normal to prolonged
TT: no effect
ECT: no effect
dTT: no effect
Source: Adapted with permission from references 35 and 39.
Careful attention to blood collection technique, sample processing, and laboratory quality control is critical for reliable coagulation test results. Blood is collected in syringes or vacuum tubes that contain heparin, EDTA, or sodium citrate. Because heparin and EDTA interfere with several clotting factors, only sodium citrate is used for coagulation and platelet tests. Errors in coagulation can be significant unless quality assurance is strict concerning specimen collection, reagents, controls, and equipment. Factors that promote clotting and interfere with coagulation studies are as follows:
Tissue trauma (searching for a vein)
Prolonged use of tourniquet
Heparin contamination from indwelling catheters
Slow blood filling into collection tube
Bleeding risk and test results
The major determinants of bleeding are the intensity of the anticoagulant effect, the underlying patient characteristics, the use of drugs that interfere with hemostasis (Tables 17-3, 17-6, and 17-8), and the length of anticoagulant therapy. When evaluating anticoagulation treatment, one must weigh the potential for decreased thrombosis risk versus increased bleeding risk. Serious bleeding can occur in patients prone to bleeding, even when the anticoagulant response is in the therapeutic range. The risk of bleeding is usually higher earlier in therapy (eg, when both heparin and warfarin are given together, which may be related to excessive anticoagulation). Also, patients who have a coexisting disease that elevates PT, aPTT, or both (eg, liver disease) are often at much higher risk of bleeding. In these patients, the use and intensity of anticoagulation that should be employed are controversial. Patient on oral anticoagulants who experience GI bleeding are more likely to receive a diagnosis of GI cancer.36,37 Thus, patients on anticoagulation medication who have GI bleeding should undergo further evaluation to assess possibility of a GI malignancy. More information about these types of tests can be found in Chapter 15.
Select Factors Altering Pharmacokinetics and Pharmacodynamics of Warfarin
ANTICOAGULANT EFFECT POTENTIATED
ANTICOAGULANT EFFECT COUNTERACTED
Low vitamin K intake
Increased vitamin K intake
Reduced vitamin K absorption in fat malabsorption
Heart failure exacerbation
Alcohol (acute consumption or binge drinking)
Alcohol (chronic consumption)
Source: Adapted with permission from reference 39.
Prothrombin Time/International Normalized Ratio
Normal range for PT: 10 to 13 seconds but varies based on reagent-instrument combinations; normal range for INR: 0.8 to 1.1; therapeutic range for INR depends on indication for anticoagulation; most indications: 2 to 3
The prothrombin time (PT), also called ProTime, test is used to assess the integrity of the extrinsic and common pathways (factors II, V, VII, X). Deficiencies or inhibitors of extrinsic and common pathway clotting factors results in a prolonged PT; however, it should be noted that the PT is more sensitive to deficiencies in the extrinsic pathway (factor VII) compared with the common pathway (factors V, X, II, and fibrinogen).38
Assay performance characteristics, standardization, and reporting
The PT is dependent on the thromboplastin source and test method used to detect clotting. Thromboplastin reagents are derived from animal or human sources and include recombinant products. Factor sensitivity is highly dependent on the source of the thromboplastin and can exhibit variability between different lots of the same reagent. Some thromboplastin reagents are less sensitive to changes in factor activity. This means that it takes a more significant decrease in factor activity to produce a prolongation of the PT. Differences in reagent sensitivity, combined with the influence of endpoint detection, affect clotting time results both in the normal and therapeutic ranges. Large differences in factor sensitivity between comparative methods can result in conflicting interpretation of results, both in the assessment of factor deficiencies and adequacy of anticoagulation therapy. Heparin also may prolong PT because it affects factor II in the common pathway; the addition of a heparin neutralizing agent to the blood sample can blunt this effect at heparin concentrations up to 2 units/mL.38 However, at higher concentrations of heparin—whether due to higher doses of heparin or sample collection issues—the neutralizing agent may not be enough, and the PT may be prolonged. These “crossover” effects may have to be considered when oral and parenteral anticoagulants are given concomitantly for several days to avoid premature discontinuation of the parenteral agent. The PT is not as sensitive as the aPTT for dabigatran; PT levels may be normal or prolonged while a patient is on dabigatran, so this is not a useful test for monitoring or measuring dabigatran levels. In terms of the oral factor Xa inhibitors, the PT is more sensitive to rivaroxaban compared with apixaban or edoxaban; therapeutic doses of rivaroxaban can result in a normal or prolonged PT level.35 The PT is not sensitive enough for assessing apixaban or edoxaban therapy.35
Because PT results can vary widely depending on the thromboplastin source, the INR is the standardized reporting method for monitoring warfarin therapy. The INR is calculated according to the following equation:
where the International Sensitivity Index (ISI) expresses the sensitivity of the thromboplastin reagent compared with the World Health Organization reference standard. The more sensitive or responsive the reagent, the lower the ISI. Theoretically, an INR result from one laboratory should be comparable to an INR result from a different laboratory although the PTs may be different. The INR should not be used as a test for DOAC monitoring because the ISI is specific for vitamin K antagonists.35,39
Although the INR system has greatly improved the standardization of the PT, one can still expect differences in INRs reported with two different methods, particularly in the upper therapeutic and supratherapeutic ranges. The greater the differences in the ISI values for two comparative methods, the more likely differences will be noted in the INR. Laboratories and anticoagulation clinics should review the performance characteristics of the PT method used to evaluate their specific patient populations and report changes in methods to healthcare professionals, particularly those monitoring anticoagulant therapy.
Monitoring warfarin therapy
Both the PT and INR may be reported when monitoring warfarin therapy, although clinically only the INR is used to adjust therapy. Warfarin exerts its anticoagulant effects by interfering with the synthesis of vitamin K-dependent clotting factors (II, VII, IX, and X) and the natural anticoagulant proteins C, S, and Z. Specifically, warfarin inhibits vitamin K-reductase and vitamin K epoxide reductase (VKOR), which blocks the activation of vitamin K to its reduced form. Reduced vitamin K is needed for the carboxylation of clotting precursors of factors II, VII, IX, and X. Noncarboxylated clotting factor precursors are nonfunctional, and thus an anticoagulated state is achieved.40 Warfarin is manufactured as a racemic mixture of (S)- and (R)-enantiomers; the S-enantiomer is more potent than the R-enantiomer at inhibiting VKOR, which is why the S-enantiomer is responsible for most of the anticoagulant effects of warfarin. The S-enantiomer is metabolized largely by CYP2C9, whereas the R-enantiomer is metabolized mostly by CYP1A2, and CYP3A4; other CYP enzymes also are involved in the metabolism of warfarin although to a lesser extent.
Current guidelines from various organizations recommend an INR of 2 to 3 for most indications.41–43 A higher INR of 2.5 to 3.5 is recommended for, but not limited to, patients with mechanical prosthetic heart valves in the mitral position and patients with recurrent thromboembolic events.44–46 Results below the therapeutic range indicate that the patient is at increased risk for clotting, and warfarin doses may need to be increased. Results above the therapeutic range indicate the patient is at risk for bleeding and warfarin doses may need to be decreased. Numerous drugs, disease states, and other factors prolong or shorten the INR in patients receiving warfarin by various mechanisms of action (Table 17-8).
Pharmacogenomics and oral anticoagulant therapy
Genetic variability in the genes coding for CYP2C9, VKOR complex subunit 1 (VKORC1), and CYP4F2 can influence warfarin dosing by altering its pharmacokinetics and pharmacodynamics.47,48 CYP2C9 and VKORC1 have a larger influence compared with CYP4F2. More information regarding CYP2C9 and VKORC1 can be found in Chapter 6. The CYP4F2 enzyme normally plays a role in the conversion of vitamin K to vitamin KH2, which is needed to carboxylate the clotting factor precursors; patients with the CYP4F2*3 variant may need higher warfarin dose requirements compared with noncarriers.48
U.S. Food and Drug Administration–approved warfarin pharmacogenomics testing devices are available, including one that is marketed as direct-to-consumer; each one tests for the CYP2C9*2 and CYP2C9*3 variants, and some may test for the VKORC1 variants.29;30 Genetic testing, if used, should be used along with patient characteristics, clinical considerations, and continued INR monitoring for optimal outcomes associated with warfarin use.
Although genetic variants have not been well studied regarding the DOACs, some potential genes may influence a patient’s response to these medications. Single nucleotide polymorphisms on the CESI and ABCB1 genes can affect peak and trough levels for dabigatran, which may be associated with rates of bleeding.49 Genetic variations in ABCB1 and CYP3A4 may alter drug levels of rivaroxaban, whereas ABCB1 and SULTA1A may alter drug levels of apixaban.49 Edoxaban was shown to have little interpatient variability due to genetic variations in the factor X, ABCB1, CYP2C9, and VKORC1 genes.49 Further studies are needed to elucidate potential genetic influences in the dosing of DOACs and associated clinical outcomes.
Activated Partial Thromboplastin Time
Normal range: varies by manufacturer, generally between 25 and 35 sec; therapeutic range for heparin-treated patients is 1.5 to 2.5 times control aPTT
The activated partial thromboplastin time (aPTT) is used to screen for deficiencies and inhibitors of the intrinsic pathway (factors VIII, IX, XI, and XII) as well as factors in the final common pathway (factors II, V, and X). The aPTT also is commonly used as a surrogate assay to monitor UFH and DTIs. The aPTT, reported as a clotting time in seconds, is determined by adding an aPTT reagent, containing phospholipids and activators, and calcium to the patient’s blood sample.
Factor and heparin sensitivity as well as the precision of the aPTT test depend both on the reagents and instrumentation. In addition, some aPTT reagents are formulated for increased sensitivity to lupus anticoagulants. Despite numerous attempts to standardize the aPTT, little progress has been made. The difficulty in part may reflect differences in opinion as to the appropriate heparin sensitivity, the need to have lupus anticoagulant sensitivity for targeted patient populations, and suitable factor sensitivity to identify deficiencies associated with increased bleeding risk. Normal and therapeutic ranges must be established for each reagent instrument combination, and ranges should be verified with changes in a lot of the same reagent. Laboratory errors may cause either prolongation or shortening of the aPTT; these may include an inappropriate amount and concentration of anticoagulant in the collection tube, time between collection of the blood specimen and performance of the assay, inappropriate collection site (ie, through a venous catheter, which contains heparin), and inappropriate timing of blood collection.50
Causes of aPTT prolongation
In addition to reagent specific issues impacting aPTT responsiveness, hereditary diseases or other acquired causes may prolong aPTT test results. Causes of aPTT prolongation include the following2,51:
Deficiency of factor VIII, IX, XI, XII, prekallikrein, or high-molecular weight kininogen (PT is normal)
Deficiency of fibrinogen or factor II, V, or X (PT also is prolonged)
Lupus anticoagulant (PT usually normal)
Heparin (PT less affected than aPTT; PT may be normal)
Bivalirudin, or argatroban (PT usually also prolonged)
Dabigatran (less accurate at higher dabigatran concentrations)
Liver dysfunction (PT affected earlier and more than aPTT)
Vitamin K deficiency (PT affected earlier and more than aPTT)
Warfarin (PT affected earlier and more than aPTT)
DIC (PT affected earlier and more than aPTT)
Specific factor inhibitors (PT normal except in the rare case of an inhibitor against fibrinogen, factor II, V, or X)
Use of aPTT to monitor heparin
Although used to detect clotting factor deficiencies, the aPTT is used primarily for monitoring therapeutic heparin therapy and may be used to qualitatively monitor dabigatran therapy. The generally accepted therapeutic range of heparin is an aPTT ratio of 1.5 to 2.5 times the control value.40 Given the interpatient and intrapatient variability that can result from aPTT reagents, alternative means of monitoring heparin therapy are being scrutinized. This 1.5 to 2.5 aPTT ratio corresponds to the following concentrations39:
A plasma heparin concentration of 0.2 to 0.4 units/mL by assay using the protamine titration method
A plasma heparin concentration of 0.3 to 0.7 units/mL by assay using the inhibition of factor Xa
Unfractionated heparin should be given by continuous IV infusion or subcutaneous injection, with exact dosing dependent on the indication. The aPTT should be drawn at baseline, 6 hours after continuous IV heparin is begun, and 6 hours after each subsequent dosage adjustment because this interval approximates the time to achieve steady-state levels of heparin. Institutions may have their own specific heparin dosing nomogram or base their nomogram on one used in clinical studies; using a nomogram also allows quick fine-tuning of anticoagulation by nurses without continuous physician input.
Activated partial thromboplastin time determinations obtained earlier than 6 hours, when a steady-state concentration of heparin has not been achieved, may be combined with heparin concentrations for dosage individualization using non–steady-state concentrations. This approach has been demonstrated to reduce the incidence of subtherapeutic aPTT ratios significantly during the first 24 hours of therapy.52,53 This finding is important because the recurrence rate of thromboembolic disease increased when aPTT values were not maintained >1.5 times patient baseline aPTT during the first 24 hours of treatment.54,55
Heparin concentration measurements may provide a target plasma therapeutic range, especially in unusual coagulation situations such as pregnancy, in which the reliability of clotting studies is questionable. In this setting, shorter-than-expected aPTT results in relation to heparin concentration measurements may be indicative of increased circulating levels of factor VIII and increased fibrinogen levels.56 Patients may have therapeutic heparin concentrations measured by whole blood protamine sulfate titration or by the plasma anti-Xa heparin assay. However, they may have aPTTs not significantly prolonged above baseline. This difference has been referred to as a dissociation between the aPTT and the heparin concentration.57 Many of these patients have short pretreatment aPTT values.
Current recommendations for patients with decreased aPTT results on heparin are that such patients be managed by monitoring heparin concentrations using a heparin assay to avoid unnecessary dosage escalation without compromising efficacy. These patients, referred to as pseudoheparin resistant, may be identified as having a poor aPTT response (to an adequate heparin concentration >0.3 units/mL via plasma anti-Xa assay) despite high doses of heparin (>50,000 units/24 hours; usual dose is 20,000 to 30,000 units/24 hours). When higher doses of heparin (>1,500 units/hour) are required to maintain therapeutic aPTT values, high concentrations of heparin-binding protein or phase reactant proteins bind and neutralize heparin. Additionally, thrombocytosis, or AT deficiency, may exist.
Another use for the aPTT is to demonstrate both efficacy and safety with LMWH, which have several indications. However, clinically, the anti-Xa levels are more routinely used for this class of medications. LMWH has a pharmacokinetic and pharmacodynamic profile, which makes routine monitoring unnecessary in most circumstances. Exceptions include special populations, such as those patients with renal failure or severe obesity who are at risk for being overdosed when weight-adjusted regimens are used. Both PT and aPTT times are not significantly prolonged at recommended doses of LMWHs. However, both efficacy and safety can be demonstrated by assaying anti-Xa levels.
Decreased aPTT levels
Although most attention has been focused on causes of prolonged aPTT levels, there is growing evidence of adverse events associated with decreased aPTT levels, including VTE, myocardial infarction (MI), hyperthyroidism, diabetes, spontaneous abortion, and death.58 Clotting factors of the intrinsic pathway, as well as vWF levels and activity, have been elevated in some patients presenting with decreased aPTT levels, which provides some evidence that patients with decreased aPTT levels are hypercoagulable.59 There is no definitive answer whether a shortened aPTT is a cause, a consequence, or just an association with these other conditions. To rule out whether a shortened aPTT is due to a laboratory error, such as inappropriate specimen collection, repeat testing should be performed.
Heparin alone has minimal anticoagulant effects; when it is combined with AT (normal range: 80% to 120%), the inhibitory action of AT on coagulation enzymes is magnified 1,000-fold, resulting in the inhibition of thrombus propagation. Patients who are AT deficient (<50%) may be difficult to anticoagulate, as seen with DIC (Minicase 2). The DIC syndrome is associated not only with obvious hemorrhage but also occult diffuse thrombosis.
Oral anticoagulant effect on aPTT
Although warfarin mildly elevates aPTT, aPTT is not used to monitor warfarin therapy. Therefore, if warfarin is started in a patient receiving heparin, the clinician should expect some elevation in aPTT. The aPTT provides qualitative information on dabigatran but not quantitative information; clinicians should note that a normal aPTT does not mean there is no clinically important dabigatran activity occurring in a patient.40 The aPTT is even less sensitive than PT for the oral factor Xa inhibitors and thus cannot be recommended for either qualitative or quantitative assessment for these agents.35,60
Activated Clotting Time
Normal range: 70 to 180 seconds but varies
Activated clotting time (ACT), also known as activated coagulation time, is frequently used to monitor heparin or DTIs when high doses are required, such as during invasive procedures like cardiopulmonary bypass graft surgery, percutaneous transluminal coronary angioplasty, PCI extracorporeal membrane oxygenation, valve replacements, and carotid endarterectomy. In most cases, an ACT is obtained from a POC machine using whole blood; thus, it may be run directly in the operating room as well as at the bedside when rapid heparinization is required (eg, hemodialysis unit, operating room, and cardiac catheterization laboratories).
Activated clotting time responsiveness remains linear in proportion to an increasing dose of heparin, whereas the aPTT has a log-linear relationship to heparin concentration. Corresponding ACT values up to 400 seconds demonstrate this dose-response relationship, but ACT lacks reproducibility for values in excess of 600 seconds as well as low concentrations of heparin. ACT test results can be influenced by the following factors61:
Platelet count and function
A Case of Disseminated Intravascular Coagulation
Teresa G., a 36-year-old woman in her third trimester of pregnancy, is hospitalized with clinical suspicion of DIC because of acute onset of respiratory failure, circulatory collapse, and shock. The following laboratory values for her are obtained:
36% to 45%
80% to 120%
QUESTION: What laboratory tests are used to determine if a patient is experiencing DIC? What are the expected laboratory results for these tests?
DISCUSSION: Laboratory findings of DIC may be highly variable, complex, and difficult to interpret. Scoring systems have been developed to aid in diagnosing DIC.70–73 Both PT and aPTT should be prolonged (and they are prolonged in this patient), but this may not always occur. Therefore, the usefulness of both PT and aPTT determinations may be helpful in making the diagnosis. TT is prolonged as expected. The platelet count is typically and dramatically decreased. Her MPV is inversely related to her decreased platelet count as expected, suggesting a hyperdestructive phenomenon versus a hypoproliferative state. Although FDPs are elevated, this rise is not solely pathognomonic for DIC. Increased d-dimer levels are strongly suggestive of DIC. AT determination reveals a considerable decrease consistent with DIC. Decreased AT is useful and reliable for diagnosis of DIC in the absence of d-dimer testing ability.
The main indication for using ACT over aPTT involves patients receiving high-dose heparin or DTIs. The DOACs prolong the ACT, but reproducibility is poor for the factor Xa inhibitors, and sensitivity is low for dabigatran; thus, this test is not recommended for qualitative or quantitative measurements of these agents.
Normal range: varies based on specific anticoagulant used for treatment of existing VTE; heparin: 0.3 to 0.7 International Units/mL; LMWH: 0.5 to 1 International Units/mL (twice daily therapeutic dosing); 1 to 2 International Units/mL (once daily therapeutic dosing); 0.2–0.5 International Units/mL (prophylactic dosing); fondaparinux, rivaroxaban, apixaban, edoxaban, betrixaban: not established
The anti-Xa level may be used to monitor LMWH when given in therapeutic doses; however, routine monitoring is not usually done because LMWH has a more predictable dose-response relationship than UFH. This assay is recommended to be drawn 4 hours after administration of a therapeutic weight-adjusted dose of LMWH, when anti-Xa activity has peaked. An effective plasma concentration range is approximately 0.5 to 1.1 plasma anti-Xa units/mL for twice-daily therapeutic subcutaneous dosing of LMWH, or 1 to 2 International Units/mL for once-daily therapeutic dosing of LMWH. The target range for prophylactic dosing is not as well defined as for therapeutic dosing, but 0.2 to 0.5 International Units/mL has been suggested.62 When ordering an anti-Xa test, it is imperative that the correct calibrator is used to ensure correct results; for example, the LMWH calibrator cannot be used to measure anti-Xa activity of fondaparinux. The anti-Xa level may be used as a quantitative assessment for the oral factor Xa inhibitors as long as the specific drug calibrator is used; if the specific calibrator is not used, the anti Xa level can only serve as a qualitative assessment test. Dabigatran has no effect on anti-Xa levels. Currently, some laboratories may not have specific calibrators for the oral factor Xa inhibitors, which limits the usefulness of this test (Minicase 3).
Normal range: 200 to 400 mg/dL (5.8 to 11.8 mmol/L)
Although the PT and aPTT are used to screen for deficiencies in the intrinsic, extrinsic, and common pathways, the fibrinogen assay is most commonly used to assess fibrinogen concentration. Fibrinogen assays are performed by adding a known amount of thrombin to a dilution of patient plasma. The fibrinogen concentration is determined by extrapolating the patient’s clotting time to a standard curve. Elevated fibrinogen levels may be related to pregnancy or acute phase reactions and may be associated with an increased risk of cardiovascular disease.63 Decreased fibrinogen is associated with DIC and hepatic cirrhosis; PT and aPTT levels also may be increased due to decreased fibrinogen levels, and patients may have symptomatic bleeding. Additionally, supratherapeutic heparin concentrations >1 unit/mL may result in falsely low fibrinogen concentration measurements. TT (discussed later) is the most sensitive test for fibrinogen deficiency, and it is prolonged when fibrinogen concentrations <100 mg/dL. However, the actual fibrinogen concentration occasionally must be determined. Fibrinogen levels are usually drawn as part of a DIC panel, to further explore reasons for an elevated PT or aPTT level, or to further evaluate unexplained bleeding in a patient.
Direct Oral Anticoagulant Monitoring
Genevieve H., a 67-year-old woman, is hospitalized with reports of chest pain and must undergo emergency surgery. Her home medications include rivaroxaban, metoprolol, atorvastatin, and lisinopril.
The following laboratory parameters are obtained prior to surgery:
QUESTION: What specific test result(s) can be used to assess this patient’s DOAC therapy? What are the expected results for these tests if a patient is taking a DOAC such as rivaroxaban?
DISCUSSION: She shows elevations in her PT, INR, and aPTT, while her TT is within normal limits. The PT can be used as a qualitative measurement for rivaroxaban; results may be normal to elevated if rivaroxaban is present in this patient’s system. An anti-Xa level calibrated for rivaroxaban is the only test that can quantitatively assess how much drug is in the patient’s system. The INR, although slightly elevated, is noncontributory to this patient’s findings given she is not on a vitamin K antagonist. The aPTT is also elevated, but this test has not been shown to be a reliable indicator of either qualitative or quantitative assessments of oral factor Xa inhibitors such as rivaroxaban. The TT is normal given this test is not affected by oral factor Xa inhibitors; the TT assesses the ability to convert fibrinogen to fibrin and is not affected by issues in the extrinsic or intrinsic pathways of the coagulation system.
Normal range: 17 to 25 seconds but varies according to thrombin concentration and reaction conditions
The thrombin time (TT), also known as thrombin clotting time, measures the time required for a plasma sample to clot after the addition of bovine or human thrombin and is compared with that of a normal plasma control. Deficiencies in both the intrinsic and extrinsic systems do not affect TT, which assesses only the final phase of the common pathway or essentially the ability to convert fibrinogen to fibrin.
Prolongation of TT may be caused by hypofibrinogenemia, dysfibrinogenemia, heparin, DTIs, or the presence of FDPs. The TT is ultrasensitive to heparin and dabigatran; therefore, it only is useful to show whether these drugs are present in the blood sample—not as a monitoring test or to quantify drug levels; a normal TT can exclude the presence of dabigatran. With thrombolytic therapy, laboratory monitoring may not prevent bleeding or ensure thrombolysis. However, some clinicians recommend measuring TT, fibrinogen, plasminogen activation, or FDPs to document that a lytic state has been achieved. Typically, TT is >120 seconds 4 to 6 hours after “adequate” thrombolytic therapy.
The dilute thrombin time (dTT) is a test that compensates for the extreme sensitivity of the TT to heparin and dabigatran by diluting the patient’s blood sample with normal plasma. Neither TT nor dTT is a useful monitoring test for the oral factor Xa inhibitors.
Ecarin Clotting Time
The ecarin clotting time (ECT) test is a specific assay for thrombin generation. It is used to monitor parenteral DTIs and can be used as a quantitative assessment for dabigatran (Table 17-7). Ecarin is a type of snake venom that can activate prothrombin; it is added to plasma, which cleaves prothrombin to meizothrombin, a serine protease similar to thrombin. Ecarin is a type of snake venom that can activate prothrombin.35 DTIs inhibit meizothrombin so the ECT can quantify the amount of DTI in the body by measuring the time for meizothrombin to convert fibrinogen into fibrin. Thus, a longer ECT corresponds to higher drug concentrations. ECT is not affected by other anticoagulants such as warfarin, heparin, or factor Xa inhibitors.
Clot Degradation Tests
Clot degradation tests are useful in assessing the process of fibrinolysis. These tests include FDPs and d-dimer, which can be used to diagnose DIC or thrombosis and monitor the safety and efficacy of thrombolytic therapy. Thrombolytics (eg, alteplase, reteplase, and tenecteplase) are exogenous agents that lyse clots already formed. They are used in the treatment of acute cerebrovascular accidents, MI, VTE, and peripheral arterial occlusion. The mechanism by which they activate fibrinolysis can variably impact circulating proteins (hence, the necessity for close monitoring to minimize bleeding complications and ensure efficacy). Numerous laboratory parameters have been evaluated for this purpose, including PT, aPTT, BT, fibrinogen, FDPs, and d-dimer. These laboratory parameters are discussed throughout this chapter and in Minicase 4.
A Patient on Thrombolytic Therapy
Alfred F., a 44-year-old man, has clinical signs and symptoms and electrocardiogram findings consistent with acute anterior-wall MI requiring PCI. However, he presents to a hospital without PCI capabilities. He receives reteplase between the transit time to the nearest hospital with PCI capability. Subsequently, he is started on a heparin infusion. The following pretherapy and posttherapy coagulation laboratory results are obtained:
80% to 120%
QUESTION: What might explain the elevated FDP? What accounts for the fall in the plasminogen level on completion of the lytic therapy? Finally, why is the d-dimer concentration not elevated in proportion to the greatly elevated FDP concentration?
DISCUSSION: The elevated posttherapy PT and aPTT are consistent with heparin therapy after receiving reteplase. The FDP concentration is elevated because reteplase resulted in fibrinogenolysis. Many FDPs are generated in this setting. By the nature of thrombolytic therapy, plasminogen is converted to plasmin, accounting for the decline in the plasminogen percentage. Because thrombolytic therapy was unsuccessful in full clot lysis (with predominate fibrinogenolysis), the d-dimer concentration is not greatly elevated. For this assay to have been more elevated, degradation products arising from cross-linked fibrin (fibrinolysis) would have had to be present. Fibrinogen concentrations should be followed periodically in patients receiving thrombolytic agents.
DIAGNOSTIC FOLLOW-UP: If TT, PT, or aPTT is prolonged and if circulating inhibitors or bleeding disorders are suspected, further tests are usually performed. These tests may include assays for specific clotting factors to determine if a specific deficiency exists. For example, hemophilia or autoimmune diseases may be associated with inhibitors such as antifactor VIII and the lupus anticoagulant.
Fibrin Degradation Products
Normal range: <10 mcg/mL or <10 mg/L but varies with assay
Excessive activation of thrombin leads to overactivation of the fibrinolytic system and increased production of fibrin degradation products (FDPs). Excessive degradation of fibrin and fibrinogen also increases FDPs. This increase can be observed with DIC or thrombolytic drugs. FDPs can be monitored during thrombolytic therapy, but they may not be predictive of clot lysis. False-positive reactions may occur in healthy women immediately before and during menstruation and in patients with advanced cirrhosis or metastatic cancer.
Normal range: <0.5 mcg/mL (<3 nmol/L) but varies with specific assay
d-dimer is a marker of thrombotic activity and is formed when thrombin initiates the transition of fibrinogen to fibrin and activates factor XIII to cross-link the fibrin formed; when plasmin digests the cross-linked fibrin, d-dimer is formed. The d-dimer test is specific for fibrin, whereas the formation of FDPs (discussed previously) may be either fibrinogen or fibrin derived following plasmin digestion (Figure 17-6).
The d-dimer is often used to help diagnose or rule out thrombosis in the initial assessment of a patient suspected of having acute thromboembolism; results are typically elevated if a patient is positive for VTE. However, d-dimer is a sensitive but nonspecific marker for VTE because other causes such as malignancy, DIC, infection, inflammation, and pregnancy also can elevate the d-dimer levels.64,65 Thus, a positive result does not necessarily confirm a diagnosis of VTE, but a negative result can help rule out VTE. Clinical correlation is essential, and further diagnostic workup is warranted with a positive test result to rule out other disorders as causes for abnormal levels. Various guidelines recommend using d-dimer as one possible diagnostic aid in patients with a low or moderate probability for first event VTE as determined by calculating the pretest probability through a validated scoring system such as the Wells or Geneva score; if the d-dimer result is positive, further testing, including imaging studies, is recommended.66,67d-dimer levels can increase by age and some studies have suggested using a different cut-off point for patients ≥50 years old. Using this approach, the cut-off for patients <50 years old remains <0.5mcg/mL, but for patients ≥50 years old the cut-off would be 10 times the patient’s age.65,68,69 Using the age-adjusted cut-offs may increase the diagnostic efficacy and specificity of the d-dimer without losing its sensitivity.65 In patients who have a high pretest probability for VTE, the d-dimer test is not recommended, and patients should have an imaging test done instead.64,66,67
In addition, to diagnose or rule out VTE, d-dimer has been used for its predictive value for recurrent thromboembolism in patients treated for first event idiopathic VTE. Recurrent VTE risk prediction models, such as DASH, Vienna, and HERDOO 2, include the d-dimer test into their calculations.65 Patients with normal levels of d-dimer 1 month after stopping anticoagulation therapy for a first-event idiopathic VTE have a lower risk for VTE recurrence, whereas elevated levels of d-dimer put patients at higher risk of VTE recurrence.65 Thus, in patients with elevated levels of d-dimer, an extended duration of anticoagulation therapy could be considered (Minicase 1).
d-dimer is also a common test used as an aid to diagnose and evaluate patients with DIC. Several scoring systems for DIC based on clinical and laboratory data have been developed, which include specific laboratory measurements such as platelet counts, PT, fibrinogen, and FDPs.70–73 The clinical and laboratory results are given specific scores, and when added up, indicate whether a patient is likely to have DIC. Although a d-dimer test is not specifically mentioned in some scoring systems, it is often used as a fibrin degradation marker, and it is a simple and quick test to perform. Table 17-9 provides a list of the laboratory parameters, including the d-dimer, used to diagnose DIC (Minicase 4).
Laboratory Differential Diagnosis of DIC
CHRONIC LIVER DISEASE
WNL to ↑
WNL to ↑
WNL to ↑
WNL to ↓
↓ = decreased; ↑ = increased; BUN = blood urea nitrogen; LFTs = liver function tests; WNL = within normal limits.
Near-Patient or Point-of-Care Testing Devices
Several point-of-care testing (POCT) devices are available for different coagulation tests, such as PT/INR, ACT, d-dimer, and platelet function tests. More information about POCT can be found in Chapter 4. POCT uses whole blood, a sample that may be more physiologically relevant and result in a more accurate assessment of true coagulation potential.
Point-of-care coagulation testing offers specific clinical advantages, especially when used to monitor antithrombotic therapy because test results can be combined with clinical presentation to make more timely decisions regarding therapeutic intervention. This is especially significant in emergency departments, cardiac catheterization laboratories, surgical settings, and critical care units, in which immediate turnaround time is essential to patient care decisions. With newer antithrombotic options, these technologies will become increasingly relevant and may aid in making decisions for major bleeding events and prior to emergency surgery. Although many of the newer drugs do not require routine monitoring, the availability of rapid interventional testing may be critical to the selection of certain therapies for target patient populations, particularly when these drugs have a long half-life or cannot be completely reversed. Concomitant therapy is being used increasingly in cardiac patients, especially during cardiac intervention; thus, the potential for thrombotic or hemorrhagic problems may be increased without the ability to rapidly confirm coagulation status, both at the initiation of therapy and at the conclusion of a procedure.
In outpatient settings, POCT may be not only clinically beneficial but also more cost-effective and convenient than central laboratory testing, particularly in oral anticoagulation clinics and home healthcare settings. Patients can be informed of their INR results and subsequent dosing instructions within minutes, which is an obvious time-saving element. Certain patient variables may limit the accuracy of results obtained from the currently available POC devices for INR monitoring. These include concurrent use of LMWH or UFH, presence of antiphospholipid antibodies, and Hct levels above or below device-specific boundaries.
Patient self-testing (PST) and patient self-management (PSM) for INR are options for properly selected and trained patients on long-term warfarin therapy. PST is when a patient tests their own INR but rely on a clinician for interpretation of results and any modifications to the current regimen. PSM is when patients test their own INR and adjust their own therapy, usually based on an algorithm, which offers more patient autonomy and control over their own dosages. Benefits seen in both PST and PSM include lower VTE recurrence, increased time in therapeutic range, and increased patient satisfaction; additionally, there was a mortality benefit seen in PSM but not PST.74 Although there are benefits to a PST/PSM model of care, including increased convenience to the patient, there also are issues that limit the widespread use of PST and PSM in the United States. These include reimbursement from insurance companies, lack of large-scale randomized trials using a U.S. population, low levels of awareness or understanding among healthcare practitioners and patients about these options, and areas with limited resources.67 Additionally, the cost-effectiveness of PST/PSM is not well defined. Higher costs are associated with the cost of the test strip as well as increased testing frequency, but this may be offset by the convenience of PST/PSM, especially for patients who live far away from testing facilities, who have difficulty with scheduled appointments, or who frequently travel.67 Appropriate patient selection is essential for PST/PSM to be effective; ideal patient candidates or their caregivers should have manual dexterity, have visual acuity, demonstrate competency to perform the test, have the confidence and ability to responsibly participate in self-care, and have the ability to complete a structured training course.74,75 The American Society of Hematology and the Anticoagulation Forum both endorse PST and/or PSM for appropriate patients.74;75
Many factors contribute to normal hemostasis, including interactions among vascular subendothelium, platelets, coagulation factors, natural anticoagulant proteins C and S, and substances that promote clot degradation, such as tissue plasminogen activator. In the clinical setting, the impact of these and other considerations must be evaluated. Disorders of platelets or clotting factors can result in bleeding, which may necessitate the monitoring of specific clotting tests.
Coagulation tests such as aPTT, ACT, and PT/INR are used to monitor heparin and warfarin therapies. Laboratory monitoring for DOACs is an emerging area with certain tests used for qualitative versus quantitative assessments of these agents. In general, coagulation tests are used for patients receiving anticoagulants, thrombolytics, and antiplatelet agents. The availability of rapid diagnostic tests to manage LMWH, DTIs, and platelet inhibitor drugs may influence the selection of these newer therapies. Other indications for routine use of these tests include primary coagulopathies and monitoring of drugs that may cause bleeding abnormalities. Finally, other available tests (eg, d-dimer and AT level determinations) may improve diagnostic assessment of patients with DIC and ensure appropriate treatment selection.
The contribution of material written by James B. Groce III, Julie B. Lemus, and Sheila M. Allen in previous editions of this book is gratefully acknowledged.
1. What is the INR in relation to PT?
ANSWER: PT results are not standardized as various reagent sources and test methods are used when performing this test among differing laboratories. The INR is a calibration method developed to standardize the reported PT results. The method considers the sensitivity of individual reagents as well as the clot detection instrument used, designated as ISI. Although INR reporting has improved the standardization of the PT, potential problems (eg, ISI calibration, sample citrate concentration) still remain. Thus, laboratories should review the performance of their individual methods used in reporting PT/INRs and inform clinicians who are interpreting these results of any changes.
2. How can the d-dimer be used for diagnosis of VTE?
ANSWER: In patients who have a low to moderate probability for a VTE, as calculated through a validated scoring system, the d-dimer can be drawn; if the result is positive, then further imaging is needed to confirm the diagnosis. The d-dimer is a sensitive but nonspecific marker for VTE, so a positive result does not confirm a diagnosis of VTE, but a negative result helps rule out VTE. In patients with high probability for VTE, the d-dimer test should not be performed; instead, imaging tests should be done. The d-dimer can be also be used as a way to help predict the risk of recurrent VTE and determine whether anticoagulation should be continued long term. In instances like this, the d-dimer should be drawn after the patient stops anticoagulation; if the d-dimer level is elevated, the patient is at increased risk of VTE recurrence and can be considered for extended duration of anticoagulation.
3. Which laboratory tests may be used to monitor DOACs?
ANSWER: No one single test can universally monitor DOAC medications. Certain tests are better suited for qualitative measurements, which show whether the medication is present in the body, whereas other tests are better suited for quantitative measurements, which show actual levels of medication in the body. Qualitative measurements of dabigatran can be done through aPTT, whereas quantitative measurements of dabigatran can be done with either the dTT or the ECT. Qualitative information for the oral factor Xa inhibitors can be measured using the PT (only for rivaroxaban), while anti-Xa levels can be used for quantitative information if calibrated to a specific oral factor Xa inhibitor; if the anti-Xa test is not specifically calibrated, then this test can only be used as a qualitative assessment for these medications.
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