Antithrombin III (AT III) is a regulatory glycoprotein in hemostasis and thrombosis. It is the functional heparin cofactor necessary for its anticoagulant action. It is involved in the control of the coagulation cascade. Levels increase in the presence of an acute inflammatory phase (hepatitis, inflammation, vitamin K deficiency, etc.). AT III deficiency may predispose to thrombosis and a poor response to heparin therapy. The level of AT III is 30% lower in serum than in plasma.