A patient is receiving warfarin therapy. Which lab should be reviewed to indicate the medication is therapeutic?

Choice A reason: Platelet counts measure the quantity of thrombocytes available for clot formation. While essential for overall coagulation assessment, warfarin does not decrease the number of platelets; rather, it interferes with the synthesis of vitamin K-dependent clotting factors. Therefore, platelet counts do not reflect warfarin's therapeutic efficacy. Choice B reason: The International Normalized Ratio (INR) is the standard metric used to monitor warfarin therapy. It provides a standardized result of the Prothrombin Time (PT), ensuring consistency across different laboratories. For most patients, a therapeutic INR range is 2.0 to 3.0, indicating sufficient anticoagulation to prevent thrombosis. Choice C reason: Hemoglobin levels measure the oxygen-carrying capacity of red blood cells. While a drop in hemoglobin might indicate an adverse effect of warfarin (such as occult bleeding or hemorrhage), it is not a direct measure of the drug's therapeutic level or its effect on the coagulation cascade. Choice D reason: Partial Thromboplastin Time (PTT) or activated PTT (aPTT) is primarily used to monitor the therapeutic effect of unfractionated heparin, which acts on the intrinsic and common pathways. It is not the appropriate test for monitoring warfarin, which acts on the extrinsic and common pathways.