What blood test is used to monitor INR levels in patients taking warfarin?

The prothrombin time (PT) test is specifically designed to monitor the effectiveness of anticoagulant therapy with warfarin. Warfarin works by inhibiting the vitamin K-dependent synthesis of certain clotting factors in the liver, which includes factors II, VII, IX, and X. By monitoring PT, healthcare providers can determine how well the blood is clotting and adjust the warfarin dosage accordingly to maintain the desired International Normalized Ratio (INR) within a therapeutic range, which is crucial for preventing thromboembolic events while minimizing the risk of bleeding.

In contrast, a complete blood count measures various components of blood, such as red blood cells, white blood cells, and platelets, but it does not provide information about clotting times related to warfarin therapy. Similarly, the activated partial thromboplastin time (APTT) is used to monitor patients on heparin anticoagulation therapy and evaluates the intrinsic and common pathways of coagulation, making it unsuitable for monitoring warfarin. Serum creatinine levels assess kidney function and are not directly related to coagulation status or the monitoring of INR levels. Thus, the prothrombin time test is vital for effectively managing patients on warfarin therapy