Which lab value is used to monitor warfarin therapy?

• A. INR
• B. aPTT
• C. ALT
• D. Creatinine

Answer: (A)

The main idea here is how warfarin’s effect is tracked in the blood. Warfarin reduces the already-made vitamin K–dependent clotting factors, which slows the clotting process via the extrinsic pathway. The test that best reflects this in a standardized way across different laboratories is the international normalized ratio (INR), derived from the prothrombin time. Using INR allows clinicians to adjust warfarin dosing to keep the level within a therapeutic range (often about 2.0–3.0 for many conditions, sometimes higher for certain mechanical heart valves). The other tests aren’t used to monitor warfarin: aPTT is for heparin monitoring, while ALT and creatinine assess liver and kidney function, respectively, not anticoagulation status.