A nurse is caring for a client who is prescribed warfarin therapy for an artificial heart valve. Which of the following laboratory values should the nurse monitor for a therapeutic effect of warfarin?

Rationale: A. PT measures the extrinsic and common coagulation pathways, which are affected by warfarin therapy. Warfarin inhibits vitamin K–dependent clotting factors (II, VII, IX, X), prolonging PT. Nurses also often monitor the international normalized ratio (INR), which standardizes PT results to guide therapeutic dosing. For clients with an artificial heart valve, the therapeutic INR range is usually 2.5–3.5. B. aPTT measures the intrinsic coagulation pathway and is primarily used to monitor heparin therapy, not warfarin. It does not reflect the anticoagulant effect of warfarin. C. Hemoglobin measures the oxygen-carrying capacity of the blood and can indicate anemia, but it does not provide information about coagulation or warfarin effectiveness. D. Bleeding time evaluates platelet function, not the effect of anticoagulants like warfarin. Warfarin primarily affects clotting factor synthesis, not platelet function.