Clotting time of plasma mixed with a thrombin solution. It is a measure of the conversion of FIBRINOGEN to FIBRIN, which is prolonged by afibrinogenemia, abnormal fibrinogen, or the presence of inhibitory substances, e.g., fibrin-fibrinogen degradation products, or heparin. batroxobin, a thrombin-like enzyme unaffected by the presence of heparin, may be used in place of thrombin.