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Protein S is a vitamin K-dependent plasma glycoprotein synthesized predominantly within the liver. Protein S is also synthesized in endothelial cells and present in platelets. As a part of the plasma anticoagulant system, protein S acts as a necessary cofactor to activated protein C (APC) in the proteolytic inactivation of procoagulant factors Va and VIIIa. About 60% of the total plasma protein S antigen circulates bound to C4b binding protein (C4b-BP), while the remainder circulates as free protein S. Only free protein S has anticoagulant activity. Acquired deficiency of protein S is much more common than hereditary protein S deficiency and is generally of unknown hemostatic significance (ie, uncertain thrombosis risk). Among the many causes of acquired protein S deficiency are: Vitamin K deficiency, Oral anticoagulant therapy, Acute illness (eg, acute thrombosis, recent surgery, or other disorder associated with acute inflammation), Liver disease, ICF/DIC, Thrombotic thrombocytopenic purpura, Pregnancy, oral contraceptive, or estrogen therapy, Nephrotic syndrome, Sickle cell anemia.
Overnight fasting is preferred. Before to sampling, it is advised that the patient stop using oral anticoagulants (such as heparin) for seven days and heparin for one day, respectively. Discontinuation should only be done with the treating physician's pe