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INR Blood Test: A Patient's Guide

written by: R. Elizabeth C. Kitchen•edited by: Emma Lloyd•updated: 2/26/2010

Has your doctor recommended an INR blood test? If so, read on to learn how it is used, when it is ordered, what the results mean, and what to avoid before the test.

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    INR blood test, also referred to as international normalized ratio, is often done in conjunction with another test known as PT, or prothrombin time. This is a blood test done on patients who are taking anti-coagulant medications. It may also be done on patients with a bleeding disorder or when a bleeding disorder is suspected.

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    How is This Test Used?

    An INR blood test is used to monitor how well a patient's blood thinning medications, such as warfarin, are working. Blood thinning medications are prescribed for a prolonged period of time to patients to prevent the formation of blood clots. Anti-coagulant medications must be watched very closely in order to maintain a balance between causing excessive bleeding and preventing blood clots.

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    When is This Test Ordered?

    INR blood test Patients taking an anti-coagulant medication will have the INR and PT regularly checked to ensure that their INR and PT is appropriately prolonged and to ensure that their medication is working properly. How often a patient needs to have this test done completely depends on them and their situation. If a patient is showing signs of a bleeding disorder, such as bleeding gums, heavy menstrual periods, blood in the urine or stool, loss of vision, nosebleeds, bruising, arthritic-type changes, or chronic anemia, this test may be ordered. Patients who are having surgery, or any other invasive medical procedure, may need to have this bleed test to make sure their blood is able to clot normally.

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    What Do the Test Results Mean?

    The majority of health and hospital laboratories adjust the PT results to the INR for patients taking anti-coagulant medications. During an INR test, basic blood-thinning needs is 2.0 to 3.0. Those who are at risk for forming clots should have an INR of approximately 2.5 to 3.5. Doctors use the INR to determine how a patient's medication should be adjusted to get the PT range that is right for them.

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    What Should I Avoid Before the Test?

    Certain foods, medications, and supplements can interfere with the results of the INR blood test. Medications that may interfere with the results include certain antibiotics, barbiturate medications, hormone-replacement therapy, and oral contraceptives. Supplements that may interfere with the results include vitamin K, in both the liquid nutrition form and the multivitamin form. Foods that may interfere with the results include pork and beef liver, soybean products, green tea, turnip greens, broccoli, kale, and chick peas. Prior to having this blood test it is important to tell your doctor about all medications, supplements, and herbs you are taking. They should let you know which foods to avoid. They will also state how long these foods, medications, and supplements should be avoided prior to the test to avoid interference.

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    Lab Tests Online. (2008). PT and INR. Retrieved on February 22, 2010 from Lab Tests Online:

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    Image Credits

    Blood Test: United States Navy – Wikimedia Commons