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?

D. The INR is the standard laboratory test used to monitor the effectiveness of warfarin therapy. Warfarin interferes with the synthesis of vitamin K-dependent clotting factors, primarily factors II, VII, IX, and X. The INR provides a standardized measure of coagulation status, allowing healthcare providers to adjust warfarin dosage to maintain therapeutic anticoagulation while minimizing the risk of bleeding or thrombosis. A. Platelets are involved in the process of blood clotting, but checking platelet levels is not specific to monitoring warfarin therapy. Platelet count may be relevant in assessing overall coagulation status, but it is not the primary laboratory value monitored for warfarin therapy. B. OPTT measures the time it takes for blood to clot after specific clotting factors are activated. While OPTT is used to monitor the effectiveness of heparin therapy, it is not routinely monitored for warfarin therapy. Warfarin primarily affects the extrinsic pathway of the coagulation cascade, and the international normalized ratio (INR) is the standard laboratory test used to monitor warfarin therapy. C. White blood cell count assesses the number of white blood cells in the blood and is used to evaluate the immune system and detect infections. Monitoring white blood cell count is not specific to warfarin therapy and is not routinely checked prior to administering warfarin.