Measurement of Protein S activity using a clotting method



Protein S (PS) is a vitamin K dependent glycoprotein which is synthesized in the liver. The balance between the free form and the C4bBP bound form of PS plays an important role because only the Free Protein S is believed to serve as a cofactor for Protein C.

In presence of its cofactor Protein S,  phospholipids and Calcium, Activated Protein C (APC) inactivates Factor Va and Factor VIIIa.

Under physiological conditions, Thrombin binds to Thrombomodulin exposed on endothelial cells and which is abundant in capillaries, and looses its procoagulant activity, whilst it acquires an anticoagulant function capable of activating PC to APC. Clotting pathways are then stopped by inactivation of Factors Va and VIIIa. This PC/PS inhibition pathway has a decreased activity if the free form of Protein S is deficient or abnormal. Congenital or acquired Protein S deficiencies are a risk factor for venous thrombosis.


Type Description
Congenital  Definciency  Type I  decreased PS activity: decreased total and free PS concentrations  – Quantitative
Type II  decreased PS activity: normal free and total PS levels – Qualitative
Type III  decreased PS activity: decreased free PS and normal total PS levels.
Acquired Deficiency In vitamin K deficiency patients or patients treatment with dicumarol/warfarin. This generates bleeding tendency rather than thrombosis.
In systemic sex hormone therapy and pregnancy, liver disease, and certain chronic infections (eg AIDS) PS concentrations will drop.
In the early stages of inflammatory diseases, Free Protein S concentration is decreased as a result of an elevation of C4b-BP.

PS deficiency is the underlying cause in a small proportion of cases with disseminated intravascular coagulation (DIC), deep vein thrombosis (DVT) and pulmonary embolism (PE). Rare deficiencies due to an acquired or transitory auto-antibody to Protein S have been reported (eg during chicken pox disease in some children).


  • Clotting Based Activity assay
  • Very high on board stability (can be frozen)
  • Excellent correlation with Free PS
  • Applications available for all analysers


The HEMOCLOT Protein S method is an APTT like clotting assay,  triggered by Factor IXa in presence of phospholipids, Calcium, and of a constant an in excess amount of Activated Protein C (APC).
In the first step, the diluted assayed plasma is mixed with Protein S deficient plasma (R1). Then, the Activator Reagent (R2), in a constant and optimised concentration, is added. Clotting is initiated by the addition of Calcium (Ca2+). Clotting time is then recorded. Protein S being the limiting factor, there is a direct relationship, between the Protein S concentration and the corresponding clotting time.

Ordering information

Product Name Assay principle Reference Package
in vitro clotting assay for the quantitative determination of Protein S (PS) in human citrated plasma, with a manual or automated method. CK041K   3 x 20 tests


Additional Assays

The Protein S concentration in normal human plasma is of about 25 µg/ml (1). About 40% (i.e. 10 µg/ml) is in the Free form and 60% (i.e. 15 µg/ml) circulates in blood as a non-covalent complex with C4b-BP. It is believed that only the Free form has an anticoagulant activity as  cofactor of Activated Protein C. Because of this the balance between the free form and the C4bBP bound form of protein S plays an important role.

Product Name Assay principle Reference Package
ZYMUTESTTM  Total Protein S
The ZYMUTEST Total Protein S Elisa kit is a one step, two-site immuno-assay, for measuring human Total Protein S in plasma, or in any fluid where Total Protein S can be present. RK021A   96 tests
The ZYMUTEST Free Protein S Elisa kit is a one step, two-site immuno-assay, for measuring human Free Protein S (the Activated Protein C cofactor) in plasma, or in any fluid where Free Protein S can be present.  RK015A  96 tests

The Total Protein S concentration in normal human plasma is usually in the range 70– 150%. The concentration is higher in males than in females. It tends to increase with age, and with blood lipid concentration.