Chapter 059. Bleeding and Thrombosis (Part 8)

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Screening Assays The most commonly used screening tests are the PT, aPTT, and platelet count. The PT assesses the factors I (fibrinogen), II (prothrombin), V, VII and X (Fig. 59-6). The PT measures the time for clot formation of the citrated plasma after recalcification and addition of thromboplastin, a mixture of TF and phospholipids. The sensitivity of the assay varies by the source of thromboplastin. To adjust for this variability, the overall sensitivity of different thromboplastins to reduction of the vitamin K–dependent clotting factors II, VII, IX, and X in anticoagulated patients is now expressed as the International Sensitivity Index...

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Nội dung Text: Chapter 059. Bleeding and Thrombosis (Part 8)

Chapter 059. Bleeding and Thrombosis (Part 8) Screening Assays The most commonly used screening tests are the PT, aPTT, and platelet count. The PT assesses the factors I (fibrinogen), II (prothrombin), V, VII and X (Fig. 59-6). The PT measures the time for clot formation of the citrated plasma after recalcification and addition of thromboplastin, a mixture of TF and phospholipids. The sensitivity of the assay varies by the source of thromboplastin. To adjust for this variability, the overall sensitivity of different thromboplastins to reduction of the vitamin K–dependent clotting factors II, VII, IX, and X in anticoagulated patients is now expressed as the International Sensitivity Index (ISI). An inverse relationship exists between the ISI and thromboplastin sensitivity. The international normalized ratio (INR) is then determined based on the formula: INR = (PTpatient/PTnormal mean)ISI. Figure 59-6
Coagulation factor activity tested in the activated partial thromboplastin time (aPTT) in red and prothrombin time (PT) in green, or both. HMWK, high- molecular-weight kininogen; PK, prekallikrein; F, factor. While the INR was developed to assess anticoagulation due to reduction of vitamin K–dependent coagulation factors, it is commonly used in the evaluation of patients with liver disease. This measure provides a system for comparing values from testing performed at different laboratories. However, as progressive liver failure is associated with variable changes in coagulation factors, the degree of prolongation of either the PT or the INR only roughly predicts the bleeding risk. Thrombin generation has been shown to be normal in many patients with mild to moderate liver dysfunction. As the PT only measures one aspect of hemostasis
affected by liver dysfunction, we likely overestimate the bleeding risk of a mildly elevated INR in this setting. The aPTT assesses the intrinsic and common coagulation pathways, factors XI, IX, VIII, X, V, II, fibrinogen, and also prekallikrein, high molecular weight kininogen and factor XII (Fig. 59-6). The aPTT reagent contains phospholipids derived from either animal or vegetable sources that function as a platelet substitute in the coagulation pathways and includes an activator of the intrinsic coagulation system, such as ellagic acid or the particulate activators kaolin, celite, or micronized silica. The phospholipid composition of aPTT reagents varies, which influences the sensitivity of individual reagents to clotting factor deficiencies and to inhibitors such as heparin and lupus anticoagulants. Thus, aPTT results will vary from one laboratory to another, and the normal range in the laboratory where the testing occurs should be used in the interpretation. Local laboratories can relate their aPTT values to therapeutic heparin anticoagulation by correlating aPTT values with direct measurements of heparin activity (anti-Xa or protamine titration assays) in samples from heparinized patients, although correlation between these assays is often poor. The aPTT reagent will vary in sensitivity to individual factor deficiencies and usually becomes prolonged with individual factor deficiencies of 30–50%. The relationship between defects in secondary hemostasis (fibrin formation) and coagulation test abnormalities is shown in Table 59-4.
Table 59-4 Hemostatic Disorders and Coagulation Test Abnormalities Prolonged activated partial thromboplastin time (aPTT) No clinical bleeding –factors XII, high-molecular-weight kininogen, protein kinase Variable, but usually mild, bleeding –factor XI, mild FVIII and FIX Frequent, severe bleeding – severe deficiencies of FVIII and FIX Heparin Prolonged prothrombin time (PT) Factor VII deficiency Vitamin K deficiency – early Warfarin anticoagulation
Prolonged aPTT and PT Factor II, V or X deficiency Vitamin K deficiency – late Direct thrombin inhibitors Prolonged thrombin time Heparin or heparin-like inhibitors Mild or no bleeding – dysfibrinogenemia Frequent, severe bleeding – afibrinogenemia Prolonged PT and/or aPTT not correct with mixing with normal plasma Bleeding – specific factor inhibitor
No symptoms, or clotting and/or pregnancy loss – lupus anticoagulant Disseminated intravascular coagulation Heparin or direct thrombin inhibitor Abnormal clot solubility Factor XIII deficiency Inhibitors or defective cross-linking Rapid clot lysis Deficiency of α2-antiplasmin or plasminogen activator inhibitor 1 Treatment with fibrinolytic therapy