What does the Antithrombin III Activity test measure?

Why is Antithrombin III Activity measured?

Antithrombin deficiency — either inherited or acquired — predisposes to venous thromboembolism and is one of the classical thrombophilias investigated after unprovoked or recurrent clots, particularly in younger patients or those with a family history. Inherited deficiency is autosomal dominant and divided into type I (quantitative reduction) and type II (dysfunctional protein), though the activity assay used clinically captures both. Acquired deficiency accompanies sepsis, disseminated intravascular coagulation, nephrotic syndrome, liver disease, and heparin therapy, all of which must be excluded before attributing a low result to a hereditary cause. Serial testing off anticoagulation and outside the acute event is essential for reliable interpretation.

Reference ranges

Standard reference

Approximately 80–120% of normal pooled plasma activity; neonatal and paediatric ranges differ.

Functional medicine optimal

Activity consistently above 80% is typical in healthy adults; values below 50–60% confer higher thrombotic risk, though clinical penetrance of inherited deficiency is incomplete and influenced by additional risk factors.

Units

%

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