NEW YORK (Reuters Health) – INR testing intervals of 4 weeks or less are typically recommended for patients on long-term warfarin therapy, but new research suggests that less frequent testing may be safe for patients with stable INR values.
“Although multiple studies have addressed the optimum target intensity of anticoagulation, few studies have addressed the optimal testing frequency,” lead author Dr. Daniel M. Witt, from Kaiser Permanente Colorado Clinical Pharmacy Anticoagulation Service, Lafayette, and colleagues explain.
The goal of the present study, reported in the July 30th issue of Blood, was to determine if researchers could identify predictors for patients who consistently have INR values within the therapeutic range. Such patients, the authors believe, might be candidates for less frequent INR monitoring.
To investigate, the researchers analyzed data from 2504 subjects with consistently therapeutic INRs during at least 6 months of regular testing and from 3569 subjects with one or more INR values that fell outside the therapeutic range during the same time frame. Depending on guidelines and indications, a therapeutic INR is typically considered 2.0 to 3.0 or 2.5 to 3.5.
Consistent with previous reports, subjects in the stable INR group had a lower combined rate of bleeding and thromboembolism than did subjects in the comparison group: 1.1% vs. 3.6% (p < 0.001).
On multivariate analysis, age older than 70 years, absence of diabetes, absence of heart failure, and not receiving estrogen therapy were significant predictors of stable INR status with odds ratios of 1.54, 1.87, 1.43, and 1.32, respectively.
Relative to comparison subjects, patients in the stable group were 52% less likely to have a target INR value of 3.0 or higher and 4% less likely to have increasing chronic disease scores.
If verified in randomized trials, the findings suggest that INR testing intervals of longer than 4 weeks may be suitable and safe for patients with consistently therapeutic values, the researchers conclude.