Also known as coagulation factor II, thrombin is a serine protease that plays a physiological role in regulating hemostasis and maintaining blood coagulation. Once converted from prothrombin, thrombin converts fibrinogen to fibrin, which, in combination with platelets from the blood, forms a clot. Thrombin–antithrombin complex (TAT) is a protein complex of thrombin and antithrombin. It is a marker of net activation of coagulation. TAT is formed in response to the high thrombin level caused by coagulation following a ruptured vessel. Since thrombin is rapidly bound by antithrombin, TAT is a useful measure for thrombin level in the blood. Thrombin can pass the blood–brain barrier, destroying neurons and potentially causing cerebral edemas. The half-life of TAT is approximately 15 minutes.