The international normalized ratio, or INR, is a test used by doctors to determine the effectiveness of treatment with blood thinning medications such as warfarin. The INR is also used to diagnose clotting abnormalities in people who bleed excessively. INR is a ratio between your prothrombin time, or PT, and the laboratory's standard PT value. The INR helps compensate for differences in testing methods among different laboratories. If you are taking warfarin, what you eat can affect your INR.
Coagulation Factors
Your liver manufactures a group of proteins called coagulation factors, several of which cannot be synthesized without the assistance of a vitamin K-dependent enzyme. As each coagulation factor is produced, vitamin K is deactivated, and another enzyme is required to recycle it. Once completed, the coagulation factors are released into your bloodstream in an inactive state. When a blood vessel is damaged, coagulation factors are activated by substances released at the site of injury, initiating a cascade of chemical reactions that leads to formation of a thrombus, or clot. INR measures how quickly a thrombus forms.
Warfarin
Warfarin, or Coumadin, is a medication used to prevent blood clots in individuals who have medical conditions, such as artificial heart valves, cancer, certain abnormal heart rhythms and immune disorders, which lead to excessive clotting. Warfarin interferes with the enzyme that recycles vitamin K, thereby inhibiting the production of new coagulation factors. If you eat foods containing vitamin K, this counteracts warfarin's effects and permits the synthesis of coagulation factors. Your INR then falls, indicating an increased tendency to form clots.
Vitamin K Foods
Many foods contain vitamin K, and your doctor may ask you to avoid them if you are taking warfarin. Green leafy vegetables are among the best sources of vitamin K. Kale, spinach, collards, chard, parsley, turnip greens and mustard greens are high in vitamin K. Animal sources of vitamin K include liver, milk, yogurt, egg yolks and fish liver oils. If you eat these foods when you are taking warfarin, your INR can fall and your risk for producing dangerous clots increases. Although alcohol does not contain vitamin K, it can increase your INR by interfering with warfarin's metabolism.
Considerations
Changes in diet typically won't affect INR in healthy people. High intake of vitamin E or calcium can interfere with vitamin K absorption, but this usually does not significantly change your INR. However, if you take warfarin and typically consume a certain amount of vitamin K, if you suddenly decrease your intake while still taking warfarin, your INR could increase, indicating an increased tendency to bleed. Ask your doctor whether you should avoid vitamin K.
References
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Approach to the Patient with a Possible Bleeding Disorder"; Mark H. Beers, M.D., Editor-in-Chief; 2006
- Warren Grant Magnuson Clinical Center: Important Information to Know when You Are Taking: Coumadin and Vitamin K
