A nurse is reviewing laboratory results for a client who is taking warfarin. Which of the following laboratory tests determines if the dose of medication is within the recommended target range?

Choice A reason: Bleeding time assesses platelet function, not warfarin’s anticoagulant effect. Warfarin inhibits vitamin K-dependent clotting factors, unrelated to platelets. Monitoring bleeding time does not reflect therapeutic anticoagulation levels, making it irrelevant for adjusting warfarin doses to prevent thrombosis or bleeding. Choice B reason: Factor VIII, deficient in hemophilia A, is not affected by warfarin, which targets vitamin K-dependent factors (II, VII, IX, X). Measuring Factor VIII does not indicate warfarin’s efficacy, as it is unrelated to the drug’s mechanism, making it unsuitable for dose monitoring. Choice C reason: aPTT monitors heparin’s effect on the intrinsic clotting pathway, not warfarin’s action on vitamin K-dependent factors. Warfarin requires INR for therapeutic monitoring, as aPTT is insensitive to its effects, making it inappropriate for assessing warfarin’s anticoagulation range in patients. Choice D reason: INR standardizes prothrombin time, measuring warfarin’s effect on vitamin K-dependent clotting factors. It ensures therapeutic anticoagulation (e.g., INR 2-3), preventing thrombosis or bleeding. INR is the gold standard for warfarin monitoring, guiding dose adjustments for safe and effective therapy.