Factor II (FII) or prothrombin is a glycoprotein synthesized by the liver, zymogen of a serine protease. It is a vitamin K-dependent clotting factor. Its half-life is 50 to 120 hours. FII is activated by the prothrombinase thrombin complex which plays a central role in the coagulation process. It will transform fibrinogen into fibrin, amplify its own formation and activate the protein C, TAFI and platelet systems. There are constitutional deficits in FII which are very rare and acquired deficits which can be observed during anti-vitamin K treatments or vitamin K deficiency, CIVD, anti-FII autoantibodies.