Patient Education Blog

How Often Should the INR be Tested?

Stephan Moll, MD writes…

Testing the INR only once every 3 months is sufficient (effective and safe) in patients on warfarin who have been shown to have stable INRs, a recent study shows [see reference below].


Traditionally, INR testing in patients on warfarin has been recommended in the U.S. to be done at least once every 4 weeks.


Recent Study [see ref]

  • Purpose:  To investigate whether INR testing every 12 weeks is as safe as testing every 4 weeks.
  • Methods:  250 patients on long-term warfarin who had stable INRs were enrolled into this study. “Stable INRs” were defined as not having required a warfarin dose change for at least 6 months. 124 patients were randomly assigned to once every 12 week testing, 126 to testing every 4 weeks. The investigators compared how often patients in the 2 treatment groups were in the desired INR range (“time in therapeutic range”), had extreme INR values, and changes in their warfarin dose. They also compared the groups in respect to occurrence of major bleeding events, blood clots, and death.
  • Results:  Both patient groups had INRs equally often in the therapeutic range. Fewer warfarin dose changes were needed in the group tested every 12 weeks. The groups did not differ in respect to occurrence of major bleeding events, blood clots, and death.
  • Conclusions:  The authors appropriately concluded that INR testing every 3 months seems to be as safe and as effective as testing every 4 weeks in patients who have been shown to have stable INRs.


Clinical Relevance

Frequency of INR testing can loosen up a little when it comes to testing patients on warfarin who have had stable INRs: testing only once every 3 months is safe and effective and an acceptable monitoring frequency in such patients.  However, patients should not change the frequency of their INR visits based on this info: don’t just skip your next appointment!  Talk with your health care provider to see if you are a candidate for less frequent monitoring.



Schulman S et al. Warfarin dose assessment every 4 weeks versus every 12 weeks in patients with stable International Normalized Ratios. N Engl J Med 2011;155:653-659.


Disclosure:  I have no financial conflict of interest.

Last updated:  Jan 31st, 2012

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2 Responses to “How Often Should the INR be Tested?”

  1. Curt Sandberg says:

    It does not appear that this study takes into account the possibility that “stable INR” patients may alter their diet or supplement intake, or even stop using some other medications, all of which can have an effect on one’s INR. Personal experience suggests that there can be major shifts in the INR, e.g., from 2.2 to 1.7, about 25%, in a very short time, i.e., less than four days.

    • Stephan Moll says:

      Self understood: An INR should be checked if a patient has (a) a change in medications or supplements, or (b) a significant longer-lasting change in diet.