Anti Thrombin III (Antigenic)

Antithrombin III antigen is a laboratory measurement of the amount of antithrombin, a natural anticoagulant that inhibits thrombin and factor Xa, present in the blood. The antigen assay quantifies protein concentration (not function); low antigen levels suggest antithrombin deficiency—congenital or acquired—and increase thrombosis risk. It’s measured by immunoassay and reported as a percentage or concentration.

Antithrombin III (antithrombin) antigen is usually reported as percent of normal; the common reference range is about 80–120% (roughly 0.8–1.2 IU/mL). Some laboratories use slightly different cutoffs (eg, 70–130% or 83–128%), so interpret results against the lab’s reference interval. Pregnancy, heparin therapy, and liver disease can alter levels.

Antithrombin III is a plasma serine protease inhibitor that inactivates key coagulation enzymes—primarily thrombin (factor IIa) and factor Xa. It also inhibits other activated serine proteases in the clotting cascade, including factors IXa, XIa, XIIa and kallikrein. Its anticoagulant activity is markedly accelerated by heparin, which enhances binding to these target proteases.

Yes. Antithrombin III is a circulating plasma glycoprotein in the serine protease inhibitor (serpin) family — a plasma globulin produced mainly by the liver. It inhibits thrombin and other coagulation proteases (notably factors Xa and IXa). Antithrombin deficiency, congenital or acquired, lowers anticoagulant activity and increases the risk of venous thrombosis; levels are assessed functionally.

Elevated antithrombin III levels are uncommon and usually have no harmful effects or increased bleeding risk. They often reflect recent heparin use, pregnancy, acute‑phase changes, or laboratory variation rather than disease. No specific treatment is required for mild increases; persistent or unexpected high results should prompt repeat testing, review of medications and clinical context, and specialist input if symptoms or diagnostic uncertainty persist.

Antithrombin III is commonly called antithrombin, abbreviated AT or ATIII, and is encoded by the SERPINC1 gene. As serpin C1, this plasma serine protease inhibitor is a major natural anticoagulant that inactivates thrombin and factor Xa; its inhibitory effect is markedly enhanced by heparin. Deficiencies or mutations in SERPINC1 lead to antithrombin deficiency and increased thrombotic risk.