Which test is used to monitor warfarin therapy?

• A. PTT
• B. Platelet Count
• C. PT
• D. Bleeding Time

Answer: (C)

Warfarin works by decreasing the production of vitamin K–dependent clotting factors (II, VII, IX, X). Because factor VII, which drives the extrinsic pathway, has a relatively short half-life, the extrinsic pathway’s clotting time—measured by the prothrombin time (PT)—changes early and reflects warfarin effect. That’s why PT, usually reported as the INR, is the test used to monitor warfarin therapy and guide dose adjustments. The PTT assesses the intrinsic pathway and is used to monitor heparin, not warfarin. Bleeding time and platelet count relate to primary hemostasis and platelets, not the anticoagulant effect of warfarin.