Review Question - QID 212361

This patient who experienced a hemarthrosis after eating citrus fruits such as grapefruit is most likely taking warfarin since warfarin is metabolized by P450 enzymes that can be inhibited by grapefruit juice. Patients would have normal coagulation test findings 1 day after initiating treatment with warfarin.

Warfarin is an anticoagulant that works by inhibiting the hepatic synthesis of vitamin K dependent coagulation factors. It performs this action by inhibiting the vitamin K epoxide reductase enzyme, which normally gamma-carboxylates clotting factors. Affected factors include II, VII, IX, and X as well as proteins C and S. It is important to note that the half-life of coagulation factors is several days long, so prothrombin time will not be elevated until several days after initiation of the drug. This property of coagulation factors means that patients can be transiently hypercoagulable and should be bridged with heparin.

Incorrect Answers:
Answer 2: Elevated prothrombin time would be seen in patients on maintenance warfarin therapy; however, prothrombin time does not elevate until several days after initiation of therapy.

Answer 3: Elevated partial thromboplastin time would be seen in patients on heparin or enoxaparin but this test is not used to monitor treatment with warfarin.

Answer 4: Elevated bleeding time would be seen in patients on antiplatelet agents but would not be affected in patients on warfarin therapy.

Answer 5: Combined elevations in prothrombin time and partial thromboplastin time would be seen in patients taking multiple coagulation medications.

Bullet Summary:
Prothrombin time does not increase immediately in patients taking warfarin due to the long half-life of coagulation factors.