One of the proteins made in the liver and endothelial cells and found in blood platelets is protein S, which depends on vitamin K. It is a component of the bodys natural anticoagulant system and works with activated protein C (APC) to inactivate procoagulant factors Va and VIIIa by proteolysis. Protein S deficiency affects venous thromboembolism patients. These three types of deficiency tests examine plasma levels of total protein S antigen, free protein S antigen, and protein S activity. This test is recommended when there are signs of recurrent miscarriage, pregnancy problems (such as preeclampsia, abruptio placentae, intrauterine growth restriction, and stillbirth), and perhaps arterial thrombosis.