Free Protein S Antigen (FPSAg)

Coagulation

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Description

Protein S is a cofactor for activated protein C and deficiency increases risk of venous thrombosis. Protein S levels must be interpreted with care because pregnancy, vitamin K antagonists such as warfarin, and liver disease all significantly reduce the free protein S level.

In health, only about 30-40% of plasma Protein S is believed to be biologically active, the remainder is in a reversible complex with C4b Binding Protein (C4Bbp, m. wt. 590,000d). Congenital deficiency of this vitamin K dependent factor has been associated with a tendency to thrombose.  

Warfarin, liver disease and pregnancy all significantly reduce free protein S levels. Women have lower levels than men, and oral contraceptives reduce the level further. Fresh thrombosis can sometimes reduce the level. The Werfen assay relies on free protein S binding to C4bBP coated particles and this appears to reduce the stability of protein S in the samples which should be less than around 6 hours old when frozen. Unlike free PS latex-based assays that rely only on antibodies, Werfen state that this assay is not affected by Rheumatoid factor. 

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Indications

As a part of the thrombophilia screen or to investigate familial protein S deficiency.

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Sample Type

Plasma (Citrate Blue) x 1

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Reference Range

See gender specific reference ranges on report, however, level is reduced and can be difficult to interpret in pregnancy.

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Turnaround Time

Within 14 days

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Testing Frequency

Weekly.

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External Notes

Warfarin, liver disease and pregnancy all significantly reduce free protein S levels. Women have lower levels than men, and oral contraceptives reduce the level further. Fresh thrombosis can sometimes reduce the level. The Werfen assay relies on free protein S binding to C4bBP coated particles and this appears to reduce the stability of protein S in the samples which should be less than around 6 hours old when frozen. Unlike free PS latex-based assays that rely only on antibodies, Werfen state that this assay is not affected by Rheumatoid factor. 

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Patient Preparation

No special requirements.

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References

Guidelines on the laboratory aspects of assays used in haemostasis and thrombosis. Mackie I, P. Cooper, A. Lawrie, S. Kitchen, E. Gray, M. Laffan. International journal of laboratory haematology. Volume 35, Issue 1, pages 1–13, February 2013.
Genetic analysis, phenotypic diagnosis, and risk of venous thrombosis in families with inherited deficiencies of protein S. Makris M, Leach M, Beauchamp NJ, Daly ME, Cooper PC, Hampton KK, Bayliss P, Peake IR, Miller GJ, Preston FE. Blood. 2000 Mar 15;95(6):1935-41.
Baglin et al; British Committee for Standards in Haematology. Clinical guidelines for testing for heritable thrombophilia. Br J Haematol. 2010 Apr;149(2):209-220.

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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.