Vitamin K and PT/INR results

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How much effect does the vitamin K content of green leafy vegetables have on prothrombin time/international normalized ratio (INR) results?

Reducing the amount of vitamin K-rich foods in the diet will increase the INR and make managing warfarin dosing more difficult. Increasing the amount of vitamin K-rich foods will lower the INR and raise the risk of clot. The size of this effect will depend on the amount of vitamin K-rich food usually consumed and the change in intake that the new amount represents. The greater the difference in the proportion of vitamin K intake compared with regular intake, the greater the change in the INR. There is no set formula. The solution to the problem is effective communication between patient and prescriber and good understanding of the vitamin K content of common foods. A list of such foods is available at Accessed March 18, 2009.—Claire Babcock O'Connell, MPH, PA-C (126-6)

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