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How often should I test my INR?

Publication Date: 
Mon, 02/03/2014

There are approximately 3 million people taking warfarin in the United States. Many of those patients are concerned about their increased bleeding risk, a potential for side effects of taking warfarin. You play a key role in reducing or preventing side effects such as bleeding or developing a blood clot.

The most important thing you can do to stay safe is check how much warfarin is in your blood through routine INR blood tests. Keeping the right amount of warfarin in your blood stream will reduce your risk of having a stroke or heart attack and also your risk of bruising or bleeding.1 It is important to check your INR (International Normalized Ratio) often; being in-range one or two times in a row doesn’t mean your next test will be in your target range.

What is the ‘Best’ Testing Frequency?

The “best” testing frequency is the one that keeps your INR test results in or very close to your target range.The closer you keep your test results in your target range the safer you will be and the better warfarin will work to protect you from developing a blood clot.2

Studies have shown that if you miss your INR target range by a large margin, your risk of bleeding or developing a blood clot can more than double.2,4 An INR test result of 5.0 or higher places you at a high risk of a major bruising or bleeding. An INR of 1.5 or lower increases your risk of developing a life-threatening blood clot. INR test results in these ranges are called “critical value” test results.

Alere Home Monitoring studied 4,501 patients to see if increasing a warfarin patients’ testing frequency could help reduced the risk of dangerous test results. The results were promising and supportive of weekly patient self testing.3

Testing frequency graph representing less critical values of INRs reported when testing INR 4 times a month

How much better was weekly testing?

The study showed weekly testing reduced patient risk by 30% over patients who tested their blood twice a month. Weekly testing was close to 50% better at keeping their INR blood test close to their target range than patients who tested their INR monthly.2,3,4

Weekly testing may not be easy or affordable if you have your blood drawn at a lab or your doctor’s office. Patients who test their INR at home often test weekly, eliminating travel to and from their lab. Monitoring your INR at home requires a prescription and the support of your doctor. Patient self-testing is covered by Medicare and most private insurance companies up to and including weekly testing.2

  1.  Bristol-Myers Squibb Company. Medication Guide for Coumadin Tablets and Coumadin for Injection (Package Insert). Princeton, NJ: Bristol-Myers Squibb Company. 2009.
  2. Center for Medicare and Medicaid Services. Decision Memo for Prothrombin Time (INR) Monitor for Home Anticoagulation Management (CAG-00087R) [Memorandum]. Baltimore, MD. 2008.
  3. Liska G, Hogan-Schlientz J, Sallee R. Does patient self testfrequency affect oral anticoagulation therapy time intherapeutic range? Presented at the 11th national conference on anticoagulant therapy, Boston: 2011.
  4. Hylek, E. An Analysis of the lowest effective intensity of prophylactic  anticoagulation for patients with non-rheumatic atrial fibrillation. N Engl J Med 1996; 335:540-546.
  5. Hylek, E. An Analysis of the lowest effective intensity of prophylactic  anticoagulation for patients with non-rheumatic atrial fibrillation. N Engl J Med 1996; 335:540-546