Introduction: Despite the emergence of new effective anticoagulation therapies, the prevention of thromboembolic events remains challenging in cases of poor medication adherence. Unfortunately, clinical prediction of future adherence has been suboptimal and electronic adherence measures are impractical. The inability of clinicians to easily and accurately identify poorly adherent patients has been an obstacle to the delivery of quality care. We examined two measures of real-time, self-reported adherence among warfarin treated patients to determine if these could be used to explain level of anticoagulation control.
Methods: The IN-RANGE2 Cohort recruited patients initiating warfarin therapy in 3 urban anticoagulation clinics. At each study visit, participants reported adherence using a 100-point Visual Analogue Scale (VAS, marking % of pills taken since prior visit on a linear scale) and 7-day recall of pill taking behavior before their current INR was revealed. Anticoagulation control was measured by between visit time in therapeutic INR range (BVTR), dichotomized at the cohort median. The longitudinal association between these adherence measurements and anticoagulation control was estimated using generalized estimating equations, controlling for clinical and demographic characteristics, prior BVTR, and warfarin dose changes.
Results: A total of 598 participants with 3204 (median 4) visits were studied. The median BVTR was 36.8% (interquartile range 0-73.9%). Mean VAS score was 96.6% (standard deviation 5.8), with participants reporting less than or equal to 80% adherence in 182 (5.7%) visits; missed pills in the past 7 days were reported in 308 (11.8%) visits. Multivariate regression analysis found poorer anticoagulation control (BVTR<36.8%) in those with a VAS score less than or equal to 80% (odds ratio [OR] 1.89, 95% confidence interval [CI] 1.12-3.18, p=0.02) and self-report of a change in adherence since last visit using 7-day recall (OR 1.55, 95% CI 1.20-2.01, p=0.001).
Conclusion: Self-reported VAS medication adherence at a clinic visit and changes in reported adherence since the last visit are independently associated with BVTR. Incorporating information from both the VAS and changes in 7-day pill recall into clinical practice could improve the quality of anticoagulation management, by helping clinicians identify poorly adherent patients who might benefit from specialized interventions. Because the association is independent of knowing the prior BVTR, these findings may be translatable to direct oral anticoagulant, medications that are not easily monitored with laboratory testing.