Which coagulation parameter is primarily affected by warfarin treatment?

Warfarin is an anticoagulant medication that functions by inhibiting the vitamin K-dependent synthesis of certain clotting factors in the liver, particularly factors II, VII, IX, and X, as well as proteins C and S. The primary laboratory test used to monitor the effect of warfarin treatment is the prothrombin time (PT), which evaluates the extrinsic pathway of coagulation.

The prothrombin time measures the time it takes for blood to clot and is sensitive to deficiencies in the vitamin K-dependent clotting factors. As warfarin inhibits these factors, PT becomes prolonged, indicating an increased time for clot formation. This effect is typically monitored using the international normalized ratio (INR), which standardizes PT results across different laboratories.

In contrast, activated partial thromboplastin time (aPTT), thrombin time, and platelet count are not primarily affected by warfarin. While aPTT can be influenced by certain anticoagulants, it is more relevant to the intrinsic and common pathways of coagulation, and hence it is not the primary parameter monitored for warfarin therapy. Thrombin time assesses the conversion of fibrinogen to fibrin and is not specifically affected by warfarin. Similarly