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International Normalized Ratio

Does this test have other names?

INR, standardized prothrombin time

What is this test?

This blood test looks to see how well your blood clots.

The international normalized ratio (INR) is a standardized number that's figured out in the lab. If you take blood thinners, also called anti-clotting drugs or anticoagulants, it's especially important to check your INR. The INR is figured out using the results of the prothrombin time (PT) test, which measures the time it takes for your blood to clot. The INR is an international standard for the PT.

Why do I need this test?

You might need this test if you take drugs that change the way your blood clots, such as warfarin (Coumadin) for heart problems or heparin to prevent a blood clot. Anti-clotting medicines are helpful if you are at risk for a stroke. But if you're in an accident and your blood doesn't clot quickly, you may be at risk of dangerous bleeding. Your doctor uses the INR to know whether your anti-clotting drugs are on target or whether you need a change in your dosage.

You may also need this test to help your doctor diagnose and manage certain health problems, such as liver disease and bleeding disorders.

What other tests might I have along with this test?

Your doctor will order the PT test first, so that the results can be used to calculate the INR. In a PT test, chemicals are added to your blood sample, and the lab measures the time in seconds that it takes to clot.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

The INR is a ratio, so it's just a number, not a number tied to time or another value. Your results will vary according to your age, the medicines you take, and any health problems you have. In general, the higher the number, the longer it takes for your blood to clot.

Your INR number should be between 2 and 3 if you are taking a blood thinner, but it could be different, depending on your condition. For instance, your INR goal if you are being treated for a mechanical heart valve will be different than your INR for a clot in your leg. Talk with your doctor about what your INR goal is and what it means for you.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Certain foods and any medicines you take may affect your results, especially if you take warfarin, heparin, or other anti-clotting drugs or are being treated for liver disease with vitamin K.

How do I get ready for this test?

You may be asked to stop eating foods that are high in vitamin K before this test. In addition, be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.