International Normalized Ratio

INR, standardized prothrombin time

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 anticlotting medicines or anticoagulants, it may be important to check your INR. The INR is found using the results of the prothrombin time (PT) test. This measures the time it takes for your blood to clot. The INR is an international standard for the PT.

You may need this test if you take medicines, such as warfarin, that change the way your blood clots. Anticlotting 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 healthcare provider uses the INR to know whether your anticlotting medicines are at the right dose or if you need to change the dose. Newer anticlotting medicines don't need INR monitoring and dose changes like warfarin does, so this lab test may not be used for those medicines.

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

Your healthcare provider 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.

Your provider may also order these tests if they are concerned about your liver function or risk of bleeding:

• Platelet count

• Prothrombin time

• Activated partial thromboplastin time

• Fibrin D-dimer

• Fibrinogen level

• Thrombin time

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

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 form a 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. The INR goal is usually higher for mechanical heart valves, especially the mitral valve. That goal is 2.5 to 3.5. Talk with your healthcare provider about what your INR goal is and what it means for you.

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

Certain foods such as green leafy vegetables and any medicines you take may affect your results. This is true if you take warfarin, heparin, or other anticlotting medicines or are being treated for liver disease with vitamin K. It's very important to follow your healthcare provider's instructions for having this test. 

You may be asked to stop eating foods that are high in vitamin K before this test. Be sure your healthcare provider 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.