Education Blog for Healthcare Professionals

How Often Should the INR be Tested?

Stephan Moll, MD writes…

Traditionally, INR testing in patients on warfarin has been recommended by anticoagulation providers in the U.S. to be done at least once every 4 weeks. However, a recent study showed that testing every 3 months is sufficient (effective and safe) in patients who have been shown to have stable INRs.


The Study

  • 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 the percentages of “time in therapeutic range” of the two treatment groups, the number of extreme INR values, and changes in warfarin doses. They also compared the groups in respect to occurrence of major bleeding events, thromboembolism, and death.
  • Results:  The percentage of “time in therapeutic range” was similar in both groups (74.1 % in the 4 week testing group, 71.6 % in the every 3 months testing group) – statistically a non-inferiority. 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

Anticoagulation providers can relax the frequency of INR testing in patients on warfarin who have stable INRs:  in these patients testing only every 3 months is safe and effective and an acceptable monitoring frequency.



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

Tags: , , ,

This entry was posted by Clot Connect on at and is filed under Uncategorized, Warfarin (Coumadin). You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

Comments are closed.