Prothrombin time
Prothrombin time (PT) is the primary assay used in monitoring oral anticoagulant therapy. The prolongation of PT depends on reductions in three of the vitamin K dependent clotting factors (II, VII and IX). Changes in the PT noted in the first few days of vitamin K antagonist (VKA) therapy are primarily due to reductions in factors VII and IX, which have the shortest half-lives (6 and 24 hours, respectively).