Which lab value is commonly used alongside INR to monitor warfarin therapy?

• A. Prothrombin Time (PT)
• B. aPTT
• C. Platelet count
• D. Creatinine

Answer: (A)

Monitoring warfarin relies on the prothrombin time because warfarin inhibits vitamin K–dependent clotting factors, which are part of the extrinsic coagulation pathway assessed by PT. The INR is a standardized version of PT, so it’s reported alongside PT to ensure consistent interpretation across labs. The aPTT checks the intrinsic pathway and is used for heparin therapy, not warfarin. Platelet count reflects platelet number and function, not the effect of vitamin K antagonists. Creatinine indicates kidney function, not anticoagulation status. So the lab value commonly used with INR to monitor warfarin therapy is the Prothrombin Time.