A nurse is caring for a client who has a new prescription for warfarin. The nurse should use the results of which of the following diagnostic tests to monitor the effect of this therapy?

A. White blood cell count (WBC): WBC count is not relevant for monitoring anticoagulation therapy. B. Activated partial thromboplastin time (aPTT): aPTT is used to monitor heparin therapy, not warfarin. C. Prothrombin time (PT): PT is the test used to monitor the effectiveness of warfarin therapy and is often expressed as an International Normalized Ratio (INR). D. Platelet count: Platelet count is not used to monitor warfarin therapy.