Human thrombin

Human thrombin (coagulation factor IIa) is a highly specific protease that transforms plasma fibrinogen into fibrin which, in the presence of clotting factor XIII in the patient's plasma, is cross-linked to form a stable clot. When applied to a surgical wound where bleeding is present, thrombin activates fibrinogen in the patient's plasma to form fibrin, which results in clot formation and hemostasis. The fibrin clot is stabilized by cross-linking occurring as a result of activation of the patient's endogenous factor XIII, which requires the presence of calcium.

Human thrombin does not require any intermediate physiological agent because it naturally clots the fibrinogen of the blood directly. Any failure to clot blood occurs in the rare case where the primary clotting defect is the absence of fibrinogen itself. The speed with which human thrombin clots blood is dependent upon the concentration of both the human thrombin used and fibrinogen present.