American Board of Internal Medicine (ABIM) Certification Practice Exam

Under what condition should vitamin K be administered to patients taking warfarin?

1. When INR is below 2.0

2. When INR is between 2.0 and 9.0

3. When INR is above 9.0

4. It should never be given

The correct answer is: When INR is above 9.0

Vitamin K administration is indicated when a patient's International Normalized Ratio (INR) exceeds 9.0, particularly in patients on warfarin therapy. Warfarin is an anticoagulant that functions by inhibiting Vitamin K epoxide reductase, an enzyme critical for the synthesis of clotting factors II, VII, IX, and X. When the INR is above 9.0, it signifies a significantly increased risk of hemorrhage due to the potent anticoagulatory effect of warfarin. In such scenarios, vitamin K acts as an antidote, effectively reversing the anticoagulation by restoring the levels of functional clotting factors that have been depleted due to warfarin usage. Administering vitamin K in this situation can help reduce the risk of bleeding complications. It is important to note that while lower INRs may require monitoring and possible adjustments to warfarin dosage, vitamin K is generally not warranted until the levels are critically elevated, as excessive or unnecessary administration can complicate further anticoagulation management. Therefore, providing vitamin K in response to an INR above 9.0 is a standard and necessary protocol to ensure patient safety and prevent adverse effects associated with extreme anticoagulation.