Protein S


4.5 mL blood added to 0.5 mL citrate.


Functional or immunoassay.

Reference Interval:

Refer to laboratory. Typically around 60-120% of the activity in pooled normal control plasma.


Investigation of heritable thrombophilia in selected patients, such as those presenting with venous thrombosis at an early age (< 40), recurrent unprovoked venous thrombosis, pregnant women at risk of venous thrombosis or those with a strong family history of unprovoked recurrent thrombosis.

Testing for heritable thrombophilia is not indicated in unselected patients presenting with venous thromboembolism.

Neonates and children with purpura fulminans.


There is some overlap between deficiency states and the normal range.

Levels are reduced by warfarin therapy.

It is important to indicate if the patient is taking warfarin at the time of the test; it may be appropriate to defer testing until the patient has ceased the drug and if a low result is obtained for a patient on warfarin repeat testing when they have ceased warfarin is recommended.

Levels may fall during pregnancy.

Repeat testing is indicated if a low level is detected – deficiency should not be diagnosed on a single abnormal result.


Bain et al. Dacie and Lewis Practical Haematology. 11th Ed. 2012.

Kessler CM and Strickland DK. Clin Chim Acta .1987; 170: 1-36.

Baglin T et al. Clinical guidelines for testing for heritable thrombophilia. BJH. 2010. 149; 209-220

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