Protein C deficiency

Protein C deficiency is a pair of rare genetic disorders where one of the clotting factors, Protein C, is deficient or abnormal. As Protein C regulates the clotting process, individuals with the condition often develop clots when no clotting is necessary and clots that do form do not break up properly. 
The condition is difficult to diagnose. The only definitive test is an immunoassay.
Although the condition is rarely fatal or even dangerous, it can cause skin necrosis when a patient is given warfarin. As such, other blood thinners must be used to treat the clots if they become problematic. Heparin is the treatment of choice, and usually must be used regularly for the rest of the patient's life. 
In severe cases, a liver transplant may correct certain types of the condition.
Protein C deficiency at Wikipedia