Warfarin Control Over Time Is Shaky Even in Those Who Are Initially Stable
Having international normalized ratio (INR) measurements consistently within the therapeutic range over a period of 6 months does not necessarily mean that such good performance will last, a new analysis shows. In fact, most patients will not be able to maintain that stability over the next year.
Among patients who had at least 80% of INR values between 2.0 and 3.0 during an initial 6-month period, only 34% met that threshold in the subsequent year, lead author Sean Pokorney, MD, MBA (Duke University Medical Center, Durham, NC), and colleagues report in a research letter published in the August 9, 2016, issue of the Journal of the American Medical Association.
Moreover, 36% of initially stable patients had at least one extreme INR measurement—less than 1.5 or greater than 4—in the following year, placing them at risk for stroke (for a low value) or life-threatening bleeding (for a high value).
A question physicians often consider is whether to switch their A-fib patients from warfarin, which has a proven track record for reducing stroke but comes with the inconvenience of regular monitoring and interactions with foods and other drugs, to one of the newer non-vitamin K antagonist oral anticoagulants (NOACs), Pokorney told TCTMD.
The issue continues to be debated, but most physicians seem to err on the side of keeping their patients on warfarin if they are doing well on it.
“This study really suggests that all patients who are eligible, even patients who have been on warfarin for a long period of time, should still have a shared decision-making conversation with their provider about what the best stroke prevention strategy is going forward,” Pokorney said.
Surprised by the Findings
The investigators looked at data from the Outcomes Registry for Better Informed Treatment of Atrial Fibrillation, which enrolled patients from 176 clinics between June 2010 and August 2011. Their analysis included 3,749 warfarin-treated patients who had sufficient INR measurements. Only 26% of patients had stable INR values in the first 6 months, defined as having at least 80% of values within the therapeutic range of 2.0 to 3.0.
Although initial stability was related to consistency of measurements in the subsequent year, its predictive ability was modest (C index 0.61). Even among patients whose INR readings were in the therapeutic range 100% of the time in the first 6 months, only 37% had stable measurements over the next year.
Results were similar when looking at time in therapeutic range rather than the proportion of INR values within the therapeutic range, Pokorney said.
“We were actually a little surprised by the findings, because the assumption was that the past performance of patients on warfarin really did predict the future performance of patients on warfarin,” he said.
Pokorney stressed the importance of getting patients with A-fib on some type of oral anticoagulation and said this study’s findings should be considered when discussing treatment options.
“We’re not saying that every patient should be on one of [the NOACs], but we have found that even patients who have done very well on warfarin in the past should at least be considered to be transitioned to one of these newer agents,” Pokorney said, noting that they have been shown to be at least as effective as and safer than warfarin.
Pokorney SD, Simon DN, Thomas L, et al. Stability of international normalized ratios in patients taking long-term warfarin therapy. JAMA. 2016;316:661-663.
- Pokorney reports receiving grant support from AstraZeneca, Gilead, and Boston Scientific and consulting fees from Boston Scientific and Medtronic, as well as serving on the advisory board for Janssen.