A Guide to INR Levels

  • If you’re taking warfarin, you’ll need to regularly get a blood test called the international normalized ratio, or INR.
  • An INR blood test checks how long it takes your blood to clot.
  • The results of your INR will help your doctor know if you need to adjust your warfarin dose.

INR blood test

Your doctor may prescribe a medicine called warfarin if you’ve been diagnosed with a blood clot (thrombosis). Some proteins that are important for blood clotting need vitamin K, which comes the foods you eat. Warfarin is a medicine that blocks the formation of a protein that uses vitamin K to make a blood clot.

Warfarin can prevent blood clots from forming, but it can also increase your risk of bleeds. People taking warfarin need to get regular blood tests to make sure that the medicine is working properly.

INR stands for international normalized ratio. It is measured with a blood test called PT-INR. PT stands for prothrombin time. 

This test measures how long it takes for your blood to clot. If your INR is too low, you could be at risk for a blood clot. If it’s too high, you could experience bleeding.

A typical INR target ranges from 2-3 but can vary from person to person.

The American Heart Association (AHA) recommends that people taking warfarin should be tested at least once a month. But some people require testing as often as twice a week, depending on how stable their INR is over time.

INR tests can be done at a lab or clinic. You may also be able to get an INR at home using an in-home INR monitoring machine and test strips. After self-testing, you can report your INR results to your doctor. Learn more about home testing here or talk to your healthcare team.

Sometimes your doctor may refer you to a specialized clinic called an anticoagulation management service (AMS). The clinicians in the AMS will check your INR and adjust your warfarin dose if needed.

There are a few ways you can try to keep your INR levels steady:

  1. Always take your medicine exactly as prescribed by your doctor. If you miss any doses, you should call your doctor and ask about how to safely restart it.
  2. Try to get your warfarin filled at the same pharmacy every time to make sure you always get the same brand.
  3. Tell your doctor if you start taking any new medicines related to your heart or for any other conditions. Some medicines can interact with warfarin.
  4. Keep track of foods that may have vitamin K.Increasing your vitamin K intake can make warfarin less effective by lowering your INR level. Foods with high levels of vitamin K include kale, broccoli, and brussels sprouts. Check this list to see high- and low-vitamin K foods.
  5. Tell your doctor if you make any other changes to your nutrition, such as starting a new diet or taking new supplements or herbs. These changes can also cause your INR level to go up or down.

If you have questions or concerns about taking warfarin, be sure to call your doctor. Pharmacists at your local hospital or retail pharmacy are also an excellent resource for questions.

Learn about INR testing at home here!


Warfarin – National Health Service
A Patient’s Guide to Taking Warfarin – American Heart Association
International Normalized Ratio (INR)StatPearls
Taking Medicines Safely as You Age – National Institute on Aging
Warfarin Card – American Heart Association

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