The standard management oral anticoagulant therapy for chronic treatment of venous thromboembolism is with the drug warfarin. Which of the following statement(s) is/are true concerning the administration of warfarin?

a. An important complication of warfarin therapy is skin necrosis in patients with protein C deficiency

b. Warfarin interferes with vitamin K dependent clotting factors II, VII, IX, X

Warfarin interferes with the vitamin K dependent clotting factors II, VII, IX and X, protein C, and protein S. An important complication of warfarin is skin necrosis with patients both with and without protein C deficiency. This syndrome usually involves full thickness skin slough over fatty areas such as the breasts and buttocks. Warfarin therapy should be monitored using the one stage prothrombin time (PT). The PT should be kept at 1.3 to 1.4 control for effective anticoagulation. At higher levels, there is a five-fold increase in the frequency of bleeding complications. Two major complications of Warfarin therapy include recurrent thrombosis and bleeding. It is recommended that Warfarin be continued four months after an initial episode of deep venous thrombosis. Between ten weeks and four to six months after deep vein thrombosis, there is a recurrent thrombosis rate of 8.3 episodes per 1000 patient months. Between four months and three years, recurrences fall to four episodes per 1000 patient-months. At four months, the risks of bleeding complications matches and exceeds the benefit from anticoagulant therapy and thus is the basis for discontinuing warfarin administration at this time.