Which laboratory finding is most consistently prolonged in Warfarin therapy?

Which laboratory finding is most consistently prolonged in Warfarin therapy?

• A. Prothrombin Time
• B. Activated Partial Thromboplastin Time
• C. Bleeding Time
• D. Platelet Count

Answer: (A)

Warfarin inhibits vitamin K–dependent clotting factors II, VII, IX, and X, which are part of the extrinsic pathway. Prothrombin Time specifically reflects this pathway, and factor VII has the shortest half-life, making PT particularly sensitive to changes as warfarin takes effect. That's why PT (and the INR, its standardized form) becomes prolonged consistently during therapy. The activated partial thromboplastin time looks at the intrinsic pathway and can be less consistently affected by warfarin. Bleeding time and platelet count measure platelet function and numbers, which warfarin does not directly alter. So, Prothrombin Time is the lab finding that is most consistently prolonged.

Warfarin inhibits vitamin K–dependent clotting factors II, VII, IX, and X, which are part of the extrinsic pathway. Prothrombin Time specifically reflects this pathway, and factor VII has the shortest half-life, making PT particularly sensitive to changes as warfarin takes effect. That's why PT (and the INR, its standardized form) becomes prolonged consistently during therapy. The activated partial thromboplastin time looks at the intrinsic pathway and can be less consistently affected by warfarin. Bleeding time and platelet count measure platelet function and numbers, which warfarin does not directly alter. So, Prothrombin Time is the lab finding that is most consistently prolonged.