A nurse is caring for a client who is receiving a continuous heparin infusion. Which of the following laboratory tests should the nurse review prior to adjusting the client's heparin?

A is correct because aPTT (activated partial thromboplastin time) measures the effectiveness of heparin therapy and guides dosage adjustments. B is incorrect because PT (prothrombin time) measures the effectiveness of warfarin therapy, not heparin. C is incorrect because INR (international normalized ratio) is a standardized version of PT that also monitors warfarin therapy, not heparin. D is incorrect because WBC count (white blood cell count) measures the body's immune response and has no relation to heparin therapy.