Antithrombin III is a natural anticoagulant protein, capable of inactivating thrombin, activated factor 9, activated factor 10, and other activated coagulation factors. Heparin accelerates the inactivation of activated factors by antithrombin III. Patients with decreased levels of antithrombin III may show a hypercoagulable state. A deficiency in antithrombin III can be congenital or acquired. Infusion of a plasma product containing antithrombin III can prevent thrombotic events.