ANTITHROMBIN III (AT III) - plasma

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Specimen: 4.5 mL blood added to 0.5 mL citrate.

Method: Functional assay, followed by immunoassay if abnormal.

Reference Interval: Method dependent; consult pathologist.

Functional: 80-120% of activity in pooled normal plasma Immunoassay: 0.2-0.4 g/L

Application: Investigation of tendency to 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.

Interpretation: Antithrombin III is a major natural inhibitor of activated coagulation factors; AT III deficiency is associated with an increased risk of venous thromboembolism. Deficiency may be inherited or acquired (eg liver disease, oestrogen therapy, nephrotic syndrome).

Reference: Hathaway WE. Semin Haematol 1991; 28: 19-23.

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