Proteins C and S are coagulation inhibitors produced in the liver. Vitamin K is essential for their synthesis. These proteins help stop clot formation. If the level or activity of one or both proteins is abnormally low, it may result in abnormal clot formation (thrombosis) particularly in the veins (phlebitis, miscarriages, disseminated intravascular coagulation, etc.). Protein C and S tests are often requested at the same time as other tests prescribed in thrombotic conditions (lupus anticoagulants, cardiolipin antibodies, beta-2 glycoprotein antibodies, etc.).
High levels or activities of protein C or S generally have no clinical significance. Normal levels indicate that coagulation control is likely normal. Low levels or activities that are consistent with abnormal clot formation can come from hereditary causes (mutations) or can be related to liver disorder (liver failure), vitamin K deficiency (dietary or anticoagulant such as warfarin) or related to protein C or S overuse (disseminated intravascular coagulation). Results may be falsely lowered if the sample is taken less than 10 days after a thrombotic episode and after stopping warfarin/Coumadin® for at least 2 weeks.