Protein S is a Vitamin K dependent protein primarily synthesized in the liver, but also in the endothelium and possibly in megakaryocytes. It functions as a cofactor for activated Protein C in the inactivation of activated Factor V, and there is an association between thrombosis and Protein S deficiencies.
» When conducting the assay, dilutions of patient plasma are mixed with Protein S depleted plasma
» Factor Xa, activated Protein C and phospholipids are added to the mixture, and finally calcium chloride is added to initiate clot formation
» The prolongation of the clotting time is directly proportional to the concentration of Protein S in the patient’s plasma