Which test is not typically used to monitor warfarin therapy?

• A. PT
• B. INR
• C. PTT
• D. ACT

Answer: (C)

Warfarin’s effect reduces vitamin K–dependent clotting factors II, VII, IX, and X, which primarily lengthen the extrinsic pathway. The standard way to monitor this is with the prothrombin time and its standardized INR, because PT/INR specifically reflect those factors and give a consistent measure of warfarin activity across labs. The partial thromboplastin time evaluates the intrinsic pathway and is more sensitive to heparin; its results don’t reliably track warfarin levels, so it’s not used for routine warfarin monitoring. Activated clotting time is a point-of-care test used during procedures with high-dose heparin, not for chronic warfarin management. Hence, the test not typically used to monitor warfarin therapy is the partial thromboplastin time.