Antithrombin Antigen (ATIIIAg)

Coagulation

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Description

When a functional deficiency of AT is suspected it is necessary to determine whether there is a type I deficiency (reduced synthesis) or type II deficiency (synthesis of an abnormal protein). When a type II deficiency is confirmed further assays can be requested to fully characterise the deficiency as different mutations may carry varying thrombotic risk.

Antithrombin is a serine protease inhibitor (SERPIN) and is an important natural anticoagulant, forming inactive complexes with thrombin, FXa and FVIIa, FIXa, FXIa and FXIIa. Antithrombin activity is greatly enhanced in the presence of heparin. A congenital deficiency of antithrombin activity is associated with increased risk of venous thromboembolism and is an autosomal dominant trait unless the defect is limited to the heparin-binding site.

Type II variants are classified as Reactive Site (RS) and Heparin Binding Site (HBS) defects as well as Pleiotropic Effects (PE) and type I deficiency. Type II PE refers to a single mutation having more than one effect, such as reduced AT antigen level in plasma and impaired heparin binding (e.g. AT Budapest 3, AT Utah).

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Indications

When a borderline or low level of antithrombin activity is detected, AT antigen is normally assayed to determine if the patient has a type I or II defect/deficiency. A repeatedly low ratio of AT activity/antigen can indicate the presence of a type II AT mutation capable of causing type II deficiency even in the absence of AT deficiency in the individual tested.

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Sample Type

Plasma (Citrate Blue) x 1

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Reference Range

Reference range indicated on report.

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Turnaround Time

Within 14 days

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Testing Frequency

Weekly

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External Notes

Pregnancy and combined oral contraceptive pill may slightly suppress AT level, whereas severe liver disease and treatment with L-asparaginase can result in significantly reduced level. On-going thrombosis especially with heparin therapy can reduce AT level. The latex-based AT antigen may be misleading in pregnancy as the result can often be erroneously, slightly reduced; these laboratory reports are assessed by a consultant Haematologists and appropriate comments added if required. The assay can (rarely) be affected by Rheumatoid factor and give high results that can be reduced by dilution. 

A Prothrombin Time (PT) will be performed on all samples.

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Patient Preparation

No Special Requirements

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References

P. C. COOPER et al, The phenotypic and genetic assessment of antithrombin deficiency . International Journal of Laboratory Hematology. Volume 33, Issue 3, pages 227–237, June 2011.
Elizabeth M. Van Cott Recommendations for clinical laboratory testing for antithrombin deficiency; Communication from the SSC of the ISTH. J Thromb Haemost. 2020 Jan;18(1):17-22.

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See Also

Antithrombin Activity

Please note: the above information is subject to change and we endeavour to keep this website up to date wherever necessary.