Which coagulation factor is NOT inhibited by warfarin?

Warfarin is an anticoagulant that primarily functions by inhibiting the vitamin K-dependent clotting factors in the liver. These factors include Factor II (prothrombin), Factor VII, Factor IX, and Factor X. By preventing the utilization of vitamin K, warfarin effectively reduces the synthesis of these factors, leading to a decrease in coagulation activity.

Factor XI, on the other hand, is not dependent on vitamin K for its synthesis. It is considered a part of the intrinsic pathway of coagulation but does not require vitamin K for its activity or formation. Therefore, warfarin does not have an effect on Factor XI, which allows it to remain functional even in the presence of warfarin therapy.

Understanding these distinctions is crucial when considering the mechanisms of anticoagulation and the factors affected by various anticoagulation therapies, particularly in clinical situations where coagulation status needs to be carefully monitored.