(See also Overview of Thrombotic Disorders.)
Protein C is a vitamin K–dependent protein, as are coagulation factors VII, IX, and X; prothrombin; and proteins S and Z. Because activated protein C (APC) degrades factors Va and VIIIa, APC is a natural plasma anticoagulant. Decreased protein C due to a genetic or an acquired cause predisposes to venous thrombosis.
Heterozygous deficiency of plasma protein C has a prevalence of 0.2 to 0.5%; about 75% of people with this defect experience a venous thromboembolism (50% by age 50).
Homozygous or doubly heterozygous deficiency causes neonatal purpura fulminans, ie, a severe neonatal type of disseminated intravascular coagulation (DIC).
Acquired decreases in protein C occur in patients with liver disease or disseminated intravascular coagulation (DIC), and during warfarin therapy.
Diagnosis is based on antigenic and functional plasma assays of protein C.
Patients with symptomatic thrombosis require anticoagulation with heparin or low molecular weight heparin, followed by warfarin. Use of the vitamin K antagonist, warfarin, as initial therapy occasionally causes thrombotic skin infarction by lowering vitamin K–dependent protein C levels before a therapeutic decrease has occurred in vitamin K–dependent coagulation factors.
It is probable, but not yet certain, that the direct oral anticoagulant (DOAC) inhibitors of either thrombin (dabigatran) or factor Xa (eg, rivaroxaban, apixaban) can be used in place of other anticoagulants for this disorder.
Neonatal purpura fulminans is fatal without replacement of protein C (using normal plasma or purified concentrate), along with anticoagulation with heparin or low molecular weight heparin.