A patient on warfarin therapy will be deficient in a functional amount of

• A. Fibrinogen and prothrombin
• B. Stable and labile factors
• C. Protein C and protein S
• D. Fletcher and Fitzgerald factors

Answer: (C)

Warfarin blocks vitamin K recycling, so proteins that need vitamin K–dependent gamma-carboxylation become less functional. This includes both the procoagulant factors II, VII, IX, and X and the natural anticoagulants, Protein C and Protein S. Because Protein C and Protein S have shorter circulating half-lives, their functional activity drops earlier, creating a transient deficiency in these anticoagulants when starting therapy. That’s why the most accurate answer is Protein C and Protein S. (Fibrinogen and prothrombin are procoagulant factors and would also be affected, but the question focuses on the vitamin K–dependent natural anticoagulants that diminish first.)