Protein C is a vitamin K-dependent proenzyme (enzyme precursor) involved in clotting and thrombolysis. Thrombolysis is the destruction or decomposition of a blood clot. Age of onset of protein C deficiency is between 15 and 30 years. Persons with this inherited disorder have an 8 to 10 times higher risk of thrombosis compared with those who don’t have the disorder. Most thromboses are deep venous thromboses; 40% are pulmonary embolisms.

Persons with abnormal functioning of protein C who do not have thrombosis are usually not given anticoagulants unless they are having major surgery or are immobilized for a prolonged period of time.

Persons with protein C-related thrombosis are usually treated with the anticoagulant heparin. Since there is a risk of recurrence, long-term therapy with warfarin or low molecular weight heparin is prescribed.

Protein S, also vitamin K-dependent, is a cofactor with protein C in anticoagulant activity. Symptoms of this deficiency usually appear when protein S levels are 50% of normal. Most events are vein thromboses. Recurrence is common, and patients with symptoms need to be treated with long-term warfarin or low molecular weight heparin.