Keywords: AT III, Antithrombin III


4.5 mL blood added to 0.5 mL citrate.


Functional assay followed by immunoassay if abnormal.

Reference Interval:

Method dependent - consult pathologist.

Functional (guide only): 80-120 % of activity in pooled normal plasma

Immunoassay (guide only): 0.2-0.4 g/L.


Investigation of  venous thromboembolism; recurrent, unexplained or with positive family history.

The test should not be performed whilst the patient is receiving heparin, as levels will be reduced. Immunoassay is not appropriate for initial testing.


Antithrombin is a major natural inhibitor of activated coagulation factors; antithrombin deficiency is associated with an increased risk of venous thromboembolism.

Deficiency may be inherited or acquired (eg, Liver disease, Nephrotic syndrome, oestrogen therapy).