TAILIEUCHUNG - 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. | 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

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