Protein C


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Subject: Protein C

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  • Protein C is a vitamin K–dependent coagulation inhibitor which, in its activated form, activated protein C (APC), down-regulates the activity of factors V and VIII through proteolysis. It is produced mainly in the liver. Congenital deficiency leads to a high incidence of venous thrombosis. Because of its short half-life, measured in hours, initiation of vitamin K antagonist therapy results in very rapid decline in the protein C level in normal individuals. In heterozygous individuals, such therapy may lead to very low levels of protein C activity—approaching 0%, with a high risk for venous thrombosis and coumarin necrosis.

  • Normal range: 70–140%.


  • Protein C functional level is examined in cases of suspected congenital thrombophilia, such as suspected in patients with unprovoked venous thromboembolism, especially when in unusual sites.

  • Determination of protein C antigen discriminates between type 1 protein C deficiency (concordant decrease of functional and immunologic assays) and type II deficiency, where the antigen level is normal. This difference has no known clinical implication.

  • Protein C should not be assayed in patients taking vitamin K antagonists.


Increased In

  • Diabetes

  • Nephrotic syndrome

  • Ischemic heart disease

  • Pregnancy

  • Oral contraceptives

  • Heparin therapy

  • Increased age

Decreased In

  • Congenital heterozygous deficiency of protein C, which is an autosomal trait with variable penetrance, with a prevalence of 1/500 individuals of European descent. Homozygous deficiency results in life-threatening massive thromboses in neonates (purpura fulminans).

  • Acquired: liver disease, vitamin K deficiency or use of vitamin K antagonists, l-asparaginase therapy, DIC, acute-phase reaction (thrombotic, inflammatory, surgical).


  • Highly elevated factor VIII levels falsely lower protein C measurements

  • Lupus anticoagulant may falsely elevate reported protein C levels