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%.
Use
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.
Interpretation
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).
Limitations
Highly elevated factor VIII levels falsely lower protein C measurements
Lupus anticoagulant may falsely elevate reported protein C levels