Study objective: To measure the extent of cardiovascular morbidity associated with nonadherence to 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) therapy.
Design: Retrospective cohort study.
Data source: Linked administrative health databases in Saskatchewan, Canada.
Patients: A total of 1221 patients aged 30-70 years who received a new prescription for a statin drug between 1994 and 2001, within 1 year of their first cardiovascular event (i.e., myocardial infarction, unstable angina, ischemic stroke, percutaneous transluminal coronary angioplasty [PTCA], or coronary artery bypass graft [CABG]).
Measurements and main results: Adherence was measured by the fill frequency (number of prescriptions filled during the observation period divided by months of observation). Patients with a fill frequency of 80% or greater were classified as adherent (661 patients); those with a fill frequency of 60% or less were classified as nonadherent (395 patients). The remaining 165 patients who had adherence rates of 61-79% were excluded from the analysis. The primary end point included a composite of myocardial infarction, unstable angina, PTCA, CABG, and death. Among 1056 patients, adherence was not associated with a reduction of the primary end point. However, patients in the adherent group were half as likely to experience a subsequent myocardial infarction as the patients in the nonadherent group (hazard ratio [HR] 0.45, 95% confidence interval [CI] 0.20-0.99, p=0.047). In patients younger than 65 years (both adherent and not), the associated reduction in myocardial infarction was even more profound (HR 0.14, 95% CI 0.04-0.46, p=0.001) and was accompanied by a trend for a lower frequency of unstable angina (HR 0.37, 95% CI 0.13-1.03, p=0.06). In patients 65 years or older (301 patients), adherence was not associated with significant changes in cardiovascular end points.
Conclusion: A detectable excess of cardiovascular morbidity appears to be associated with nonadherence to statin therapy. Our analysis suggests that many occurrences of myocardial infarction could be prevented with improvements in adherence. Larger studies are necessary to determine the association between adherence and other cardiovascular end points.