Protein C Antigen

The Protein C Antigen test (also referred to as Protein C or coagulation protein C antigen) is used to confirm and characterize protein C deficiency and to help separate congenital deficiency from acquired reduction. It is incorporated into thrombophilia assessments for venous thromboembolism and for arterial events at an early age, and it is applied in the workup of thrombotic complications during pregnancy. Testing also contributes to the evaluation of severe neonatal presentations suggestive of homozygous deficiency. This assay may be ordered when thrombosis occurs during vitamin K antagonist therapy and in clinical contexts known to lower protein C concentrations, including vitamin K deficiency, liver disease with decreased synthesis, sepsis or severe infection, and malignancy.

Protein C is a central regulator of anticoagulation. Thrombin bound to thrombomodulin activates protein C, and the activated form, with protein S as a cofactor, inactivates factors Va and VIIIa. This attenuates thrombin generation and secondarily facilitates fibrinolysis. Congenital heterozygous deficiency is associated with venous thrombosis and may produce antigen values that are low or in the low-normal range. Homozygous or compound heterozygous deficiency typically manifests in the neonatal period with purpura fulminans and disseminated intravascular coagulation. Protein C concentrations fall during vitamin K antagonist therapy and during periods of coagulation activation, including the immediate interval after an acute thrombotic event, which limits the utility of antigen testing for diagnosing congenital deficiency in those settings. Acquired decreases also occur with vitamin K deficiency, impaired hepatic synthesis, severe infection or sepsis, and malignancy.