Antithrombin (ATIII)

Coagulation


Description

AT functions as an important natural anticoagulant forming inactive complexes with thrombin, factor IXa and factor Xa.

Antithrombin is a serine protease inhibitor (SERPIN) synthesised in the liver that functions as a natural anticoagulant by inhibiting several activated clotting factors, the most important of which are thrombin and FXa. Two major subtypes of AT are recognised; type I is a quantitative reduction of qualitatively normal AT, and type II is due to the production of qualitatively abnormal form of AT. A congenital deficiency is associated with a thrombotic tendency and is often an autosomal dominant trait.

This thrombin-based assay detects some variants which are undetected by factor Xa-based assays although some variants with reduced activity may yield results within reference range and the ratio of AT activity / AT antigen increases sensitivity to these defects.

The Berichrom chromogenic assay (thrombin-based) measures antithrombin as heparin co-factor, and detects most Reactive Site (RS) and Heparin Binding Site (HBS) defects as well as Pleiotropic Effects (PE) and type I deficiency. The Berichrom assay has been optimized for maximum sensitivity to type II defects by reducing incubation time with thrombin to 30 seconds.


Indications

As a part of the thrombophilia screen or to investigate antithrombin deficiency.


Sample Type

Plasma (Citrate Blue) x 1.


Reference Range

Indicated on report


Turnaround Time

Within 2 weeks


Testing Frequency

Weekly or as required


External Notes

Further tests including FXa-based AT assay, progressive AT assay (AT assay in absence of heparin) and heparin-binding studies may be appropriate if type II AT deficiency is identified and tests requested by a Consultant haematologist. Genetic sequencing of the antithrombin gene (SERPINC1) can be requested from the Sheffield Genetics Service at Sheffield Children’s Hospital.

An unexpected low level will trigger assay of AT antigen and further tests may be performed if requested by a Consultant Haematologist. 


Patient Preparation

No Special Requirements.


References

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.


See Also

Thrombophilia Screen

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

Your contact for this test

team

Kieron Hickey

Thrombophilia Section Lead and Deputy Laboratory Manager - Coagulation

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Antithrombin (ATIII)