Coagulation screening tests - Investigations and procedures

Coagulation screening tests

The basic coagulation cascade (excluding co factors) is outlined below; the cascade style of reaction allows a small stimulus of negative charge contact (such as collagen) or the release of thromboplastin from the tissues to create a large amount of fibrin product (see Fig. 12.1).

Factors II (prothrombin), VII, IX and X require vitamin K for their synthesis. The coagulation screen is made up of a combination of tests:

·        The thrombin time (TT) is initiated by adding thrombin to a sample and thus assesses deficiencies/dysfunctions in fibrinogen. Fibrinogen levels and fibrin degradation (D-dimers) products can also be measured as a measure of intravascular clot break-down, e.g. disseminated intravascular coagulation or pulmonary embolism.

·        The prothrombin (PT) time is initiated by the addition of thromboplastin and thus measures the extrinsic and final common pathway. It is prolonged in deficiencies of factors VII, V, X or II. It is also prolonged in liver disease and in patients taking warfarin.

·        The activated partial thromboplastin time (APTT) or partial thromboplastin time with kaolin (PTTK) is initiated by adding an activator such as kaolin and thus measures the intrinsic and final common pathway. It is prolonged in deficiencies of factors XII, XI, IX, VIII, X or V.

·        If the coagulation times are prolonged, a 50:50 mix of patients and normal plasma is made. If such a mixture does not correct the time then the result is suggestive of the presence of an inhibitor of coagulation rather than a factor deficiency. If heparin is suspected as the cause of a prolonged TT then reptilase or protamine is added to the sample, which reverses the effects of heparin.