prothrombin thrombin fibrinogen fibrin cross-linked fibrin
Fig. 11-5. A conceptualization of commonly used screening tests of coagulation and the coagulation parameters they measure. PTT, partial thromboplastin time; PT, prothrombin time.
• Partial thromboplastin time (PTT) assay (assesses the intrinsic system): utilizes a phospholipid reagent, a particulate activator (e.g., ellagic acid, kaolin, silica, soy extract), and calcium chloride to start the enzyme reaction that leads to the formation of thrombin via the intrinsic pathway. Common confirmatory coagulation assays:
Fibrinogen: quantitative measurement of fibrinogen, useful when both the PT and the PTT are prolonged.
Thrombin time: prolonged when fibrinogen is reduced or abnormal, in the presence of inhibitors (fibrin degradation products, D dimers), and in the presence of thrombin-inhibiting drugs. Useful when both the PT and PTT are prolonged.
Mixing studies (performed to evaluate a prolonged PT or PTT): the respective assay is performed following addition of normal plasma to patient plasma. Normalization indicates a clotting factor deficiency that was corrected by addition of normal plasma. Continued prolongation indicates presence of a coagulation inhibitor. Such inhibitors may be physiologically relevant or only of in vitro significance.
Clotting factor activity assays: performed to identify clotting factor deficiencies if mixing studies normalize. FXII, FXI, FIX, and FVIII assays are useful if the PTT normalizes in mixing studies. The FVII assay is useful if the PT normalizes in mixing studies. FX, FV, FII, and fibrinogen assays are useful if both PT and PTT normalize in mixing studies. (Note: Factor XIII deficiency does not result in prolongation of the PT or PTT.)
von Willebrand antigen: quantitative assay for von Willebrand factor, useful when bleeding time or PFA-100 closure time is prolonged or when von Willebrand disease is suspected.
Table 11-4. Coagulation Tests and Normal Values
Template bleeding time (min) Platelet retention (%) Platelet aggregation Platelet factor 3 availability Prothrombin consumption (s) Clot retraction Clot solubility" Intrinsic system
Activated partial thromboplastin time (s) Extrinsic system
Prothrombin time (s) Stypven time6 Factor assays Thrombin time(s)c Reptilase time (s)c Fibrinolytic system
Staphylococcal clumping (|ig/ml)d Thrombo-Wellcotest (|ig/ml)d D-Dimer Wellcotestd
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