Antithrombin (AT) is an anticoagulant produced in the liver that inhibits thrombin, factor Xa, and other coagulation factors. AT deficiency can be either inherited (rare) or acquired, and it increases risk of venous thromboembolism (VTE). Prevalence of AT deficiency is approximately 0.02%-0.2% with no sex predominance.

Inherited AT deficiency results from AT gene mutation (SERPINC1, among others) that shows autosomal dominant inheritance. Inherited AT deficiency is further categorized into 2 subtypes. Type 1 deficiency refers to a mutation that causes reduced production of the protein. Type 2 deficiency refers to mutations that alter protein function. Type 2 defects result from mutation in the thrombin-binding site (reactive site defect, or RS), mutation in the heparin-binding site (heparin-binding site defect, or HBS; also the most common inherited type), or mutation at the carboxy-terminal end of the enzyme inhibitor (pleiotropic effect, or PE).

Acquired AT deficiency results from impaired production (due to liver disease, warfarin use, nephrotic syndrome, etc) or consumption (eg, acute thrombosis, disseminated intravascular coagulation). Other causes for reduced AT levels include extracorporeal membrane oxygenation (ECMO), hemodialysis, major surgery, asparaginase chemotherapy, oral contraceptive use, pregnancy-induced hypertension, preeclampsia, and eclampsia.