A nurse is caring for a client who is prescribed warfarin therapy for an artificial heart valve. Which of the following laboratory values should the nurse monitor for a therapeutic effect of warfarin?

A. Prothrombin time (PT): Warfarin works by inhibiting vitamin K–dependent clotting factors, which prolongs the PT. Monitoring PT, often reported as the INR (international normalized ratio), is the standard for assessing the therapeutic effect and dose adjustment of warfarin therapy. B. Hemoglobin (Hb): Hemoglobin levels help detect anemia or blood loss, which may indicate bleeding complications. However, Hb does not measure the effectiveness of anticoagulation and is therefore not the primary value to monitor for therapeutic effect. C. Activated partial thromboplastin time (aPTT): aPTT is used to monitor heparin therapy, not warfarin. Although both are anticoagulants, they affect different clotting pathways, and aPTT does not reliably indicate warfarin’s therapeutic effect. D. Bleeding time: Bleeding time evaluates platelet function and the ability to form clots at the capillary level. Warfarin primarily affects the coagulation cascade rather than platelet activity, so bleeding time is not a useful measure of its therapeutic effect.