Anti-thrombin III is a glycoprotein which is synthesized in the liver and which acts as a physiological inhibitor of thrombin. Its activity is potentiated by heparin. Potentiation of its activity is the principle mechanism by which heparin results in anticoagulation. There are two types of Anti-thrombin III deficiency: Type I is a simple deficiency of the enzyme, and both antigen and activity levels are similarly low. Type II is related to reduced enzyme activity. This test is useful in the diagnosis of Congenital Anti-thrombin III deficiency; an autosomal dominant disorder in which an individual inherits one copy of a defective gene. This condition leads to increased risk of venous and arterial thrombosis. LOW levels are also associated with Acquired anti-thrombin deficiency which is primarily due to consumption. It is observed in situations in which the activation of the coagulation system is inappropriate, such as: Disseminated Intravascular coagulation (DIC), microangiopathic hemolytic anemia, and Venoocclusive disease (VOD) in patients undergoing bone marrow transplant. See also Protein C – Immunological level, and Protein S – Total & Free levels.