What does the Antithrombin III Antigen test measure?

Why is Antithrombin III Antigen measured?

Antithrombin III deficiency — whether hereditary or acquired — is one of the inherited thrombophilias and a well-established risk factor for venous thromboembolism. Hereditary deficiency is rare, autosomal dominant, and associated with recurrent deep-vein thrombosis and pulmonary embolism, often presenting at a young age. Acquired deficiency is more common and seen during heparin therapy (consumption), in liver disease (reduced synthesis), nephrotic syndrome (urinary loss), disseminated intravascular coagulation, and acute thrombosis. The antigen test is ordered as part of a thrombophilia screen and to investigate heparin resistance, where low antithrombin blunts anticoagulant response.

Reference ranges

Standard reference

Approximately 80–130% activity or 20–30 mg/dL antigen, age- and assay-dependent.

Functional medicine optimal

Most practitioners target the laboratory reference range (80–130%); values below 70% are considered deficient and warrant further investigation, particularly when recurrent thrombosis or a family history is present.

Units

%, mg/dL

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.