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?

A. Factor VIII evaluates for hemophilia A and is unrelated to monitoring warfarin therapy. B. Bleeding time assesses platelet function and vascular integrity, not the effects of anticoagulant therapy. C. INR (International Normalized Ratio) is the correct test to evaluate therapeutic effectiveness of warfarin. The typical therapeutic range is 2.0–3.0 (or 2.5–3.5 for mechanical heart valves). It standardizes PT results across laboratories. D. aPTT monitors heparin therapy, not warfarin. Warfarin affects vitamin K–dependent clotting factors measured by PT/INR.