A nurse is caring for a client who has a prescription for warfarin. Which of the following laboratory tests should the nurse monitor?

Warfarin is an oral anticoagulant that works by inhibiting vitamin K–dependent clotting factors (II, VII, IX, and X), thereby reducing blood clot formation. Because its therapeutic and toxic effects are closely related to coagulation status, careful laboratory monitoring is essential to maintain effectiveness while preventing bleeding complications. The primary measure used to assess warfarin therapy is a coagulation test that reflects clotting time and anticoagulant effect. Nurses play a key role in monitoring these values to ensure patient safety. Rationale: A. Prothrombin time (PT) is the correct laboratory test used to monitor warfarin therapy because it measures the time it takes blood to clot via the extrinsic pathway. The PT is standardized and reported as the International Normalized Ratio (INR), which guides dosage adjustments to maintain therapeutic anticoagulation and prevent bleeding or clotting complications. B. Arterial blood gases (ABGs) assess oxygenation, ventilation, and acid-base balance. They are not related to coagulation status and therefore are not used to monitor the effectiveness or safety of warfarin therapy. ABGs are typically used in respiratory or metabolic disorders. C. Triiodothyronine (T3) is a thyroid hormone level used to assess thyroid function. It has no relationship to anticoagulation therapy or clotting status. Monitoring T3 would be relevant in thyroid disorders, not in clients receiving warfarin. D. Blood urea nitrogen (BUN) measures renal function and hydration status. While kidney function may indirectly influence medication clearance, BUN is not used to monitor the therapeutic effect of warfarin. The primary concern with warfarin is coagulation status, not renal function.