The active coagulation enzyme, thrombin, acts on soluble fibrinogen to convert it to insoluble fibrin. This reaction is manifested by the appearance of a visible fibrin clot. Any substance that interferes with this reaction, i.e. heparin or fibrin degradation products, may produce a prolonged thrombin clotting time. Low plasma fibrinogen levels as well as abnormal fibrinogen molecules seen in severe liver disease or as congenital abnormalities will also prolong the clotting time.
The Thrombin Time test is performed by adding 1 volume of Thrombin Time reagent to 1 volume of citrated plasma pre-warmed to 37C, and the time taken to detect the clot end point is recorded.