Health Guide

International normalized ratio

What is this test?

International normalized ratio (INR) is a calculation made to standardize prothrombin time. INR is based on the ratio of the patient's prothrombin time and the normal mean prothrombin time. Prothrombin time is a test to learn how fast the blood clots in patients receiving oral anticoagulant medication. This test may be used in patients with prosthetic heart valves, venous thromboembolism, or antiphospholipid syndrome[1][2][3][4][5][6][7][8][9]. A blood sample is collected for this test[6][9].

What are other names for this test?

  • INR - International normalized ratio

What are related tests?

Why do I need this test?

Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. You may need this test if you have:

  • Antiphospholipid syndrome
  • Atrial fibrillation
  • Heart valve replacement - prosthesis
  • Thrombophilia
  • Venous thromboembolism

When and how often should I have this test?

When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.

Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.

The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.

Tell your healthcare worker if you start or stop using a herbal supplement, as this test may need to be done[1].

How should I get ready for the test?

Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.

How is the test done?

A sample of blood from a vein or an artery may be collected for this test.

When a blood sample from a vein is needed, a vein in your arm is usually selected. A tourniquet (large rubber strap) may be secured above the vein. The skin over the vein will be cleaned, and a needle will be inserted. You will be asked to hold very still while your blood is collected. Blood will be collected into one or more tubes, and the tourniquet will be removed. When enough blood has been collected, the healthcare worker will take the needle out.

Before an arterial blood draw, the healthcare worker may check blood circulation to your arm or leg. Tell the healthcare worker if you use supplemental oxygen as this may affect the test results.

An artery in the wrist is the most common site to draw arterial blood. However, an artery at the inside bend of the arm or groin may be used. The area will be swabbed clean with antiseptic. Anesthetic may be used to numb the skin over the area where the blood will be drawn. A needle is used to puncture the skin. You will be asked to hold very still while your blood is collected. When enough blood is collected, the needle will be removed.

If you have an existing arterial catheter, a blood sample may be collected from that catheter.

Ask your healthcare worker if an INR testing device that can be used at home is an option for you[10][11][4][12].

How will the test feel?

The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.

During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.

If you receive anesthetic to numb the skin before an arterial blood draw, you may feel discomfort at the location where the anesthetic needle is inserted into the skin. You may feel discomfort as the needle used to draw your blood is inserted into the skin, and you may have cramping at the site during the procedure. Inform the person doing the test if you feel faint or nauseated or if the discomfort is severe.

If blood is collected from an existing catheter inserted into an artery, you will feel little or no discomfort.

What should I do after the test?

After a blood sample is collected from your vein, a bandage, cotton ball, or gauze may be placed on the area where the needle was inserted. You may be asked to apply pressure to the area. Avoid strenuous exercise immediately after your blood draw. Contact your healthcare worker if you feel pain or see redness, swelling, or discharge from the puncture site.

If blood was drawn from an artery in your arm or leg, cotton will be placed over the site and held firmly for at least five minutes to stop the bleeding. Bleeding may continue beyond five minutes if you are using medications or supplements that thin your blood or have a medical condition that causes excessive bleeding. When bleeding has stopped, a bandage will be placed firmly over the site and should be left on for 30 to 60 minutes. You should rest for at least 15 minutes after the test. Avoid heavy use of the arm or leg from which the blood was drawn for 24 hours after the blood is collected.

If blood was drawn from an existing catheter inserted into an artery, there are no special instructions for you after this test.

What are the risks?

Blood: During a blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this test.

Arterial blood: During an arterial blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once.

If you have a medical condition, or are using a medication or supplement that causes excessive bleeding, you are at a higher risk of bleeding from the puncture site. Rarely, damage to the artery that affects blood flow to the arm or the leg may occur. Talk to your healthcare worker if you have any concerns about the risks of this test.

What are normal results for this test?

Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following are considered to be normal results for this test:

  • Therapeutic Range [9]:
    • Prevention of venous thromboembolism (VTE): 2-3
    • Treatment of VTE, pulmonary embolus, valvular heart disease: 2-3
    • Treatment of arterial thromboembolism, mechanical heart valves, recurrent systemic embolism: 3-4.5

What might affect my test results?

Drug Therapy Use:

Some medications may affect the results of the test. These medications include:

  • Oritavancin (Intravenous route, Powder for Solution)
  • Hydroxocobalamin (Intravenous route, Powder for Solution)
  • Daptomycin (Intravenous route, Powder for Solution)

What follow up should I do after this test?

Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.

Where can I get more information?

Related Companies

  • National Alliance for Thrombosis and Thrombophilia -
  • American Heart Association -
  • National Hemophilia Foundation -

Related Drugs

  • Warfarin (Oral route, Tablet)


[1] Ansell J, Hirsh J, Poller L, et al: The pharmacology and management of the vitamin K antagonists. Chest 2004; 126 (Suppl 3):204S-233S.

[2] Salem DN, Stein PD, Al-Ahmed A, et al: Antithrombotic therapy in valvular heart disease - native and prosthetic: the seventh ACCP conference on antithrombotic and thrombolytic therapy. Chest 2004; 126:457S-482S.

[3] Buller HR, Agnelli G, Hull RD, et al: Antithrombotic therapy for venous thromboembolic disease: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. Chest 2004; 126(3 Suppl)::401S-428S.

[4] Ansell J, Hirsh J, Dalen J, et al: Managing oral anticoagulant therapy.. Chest 2001; 119(1 Suppl):22S-38S.

[5] Henry JB: Clinical Diagnosis and Management by Laboratory Methods, 20th ed. Saunders, 2001.

[6] Nicoll D, McPhee SJ, Pignone M, et al (Eds): Pocket Guide to Diagnostic Tests, 3rd. McGraw-Hill, New York, NY, United States, 2001.

[7] Riley RS, Rowe D, & Fisher LM: Clinical utilization of the international normalized ratio (INR). J Clin Lab Anal 2000; 14(3):101-14.

[8] Haemostatis and Thrombosis Task Force of the British Society for Standards in Haematology: Guidelines on oral anticoagulation: third edition. Br J Haematol 1998; 101(2):374-287.

[9] Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.

[10] Sunderji R, Gin K, Shalansky K, et al: Clinical impact of point-of-care vs laboratory measurement of anticoagulation. Am J Clin Pathol 2005; 123(2):184-8.

[11] Poller L, Keown M, Chauhan N, et al: Reliability of international normalised ratios from two point of care test systems: comparison with conventional methods. BMJ 2003; 327(7405):30.

[12] Oral Coagulation Monitoring Study Group: Point-of-care prothrombin time measurement for professional and patient self-testing use. A multicenter clinical experience. Oral Anticoagulation Monitoring Study Group. Am J Clin Pathol 2001; 115(2):288-96.

Last Updated: 7/4/2018
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