Which laboratory test is used to monitor warfarin therapy?

• A. International normalized ratio
• B. Prothrombin time
• C. Partial thromboplastin time
• D. Bleeding time

Answer: (A)

Warfarin dampens activity of vitamin K–dependent clotting factors, so monitoring focuses on the extrinsic pathway. The prothrombin time (PT) reflects this pathway, but PT results can vary widely from lab to lab due to different reagents. The International Normalized Ratio standardizes PT results across labs, yielding a consistent measure to guide warfarin dosing. That standardization is why INR is the test used to monitor warfarin therapy.

The partial thromboplastin time assesses the intrinsic pathway and is mainly used to monitor heparin therapy, not warfarin. Bleeding time is an outdated test with little utility for monitoring anticoagulation.