A nurse is preparing to administer a daily dose of warfarin to a client. Which of the following laboratory values should the nurse check prior to administration?

The correct answer is D: INR. The International Normalized Ratio (INR) is a crucial laboratory value to monitor when administering warfarin, an anticoagulant. INR measures the effectiveness of warfarin therapy in preventing blood clots. Monitoring INR levels helps ensure the client is receiving the appropriate dose to achieve the desired anticoagulant effect while minimizing the risk of bleeding. Checking platelets (A) is important for assessing clotting ability but not specific to warfarin therapy. The activated partial thromboplastin time (APTT) (B) is more relevant for monitoring heparin therapy. White blood cell count (C) is not directly related to warfarin therapy.