Background: Pharmacy claims data are commonly used to estimate drug exposure. We previously identified misclassification in days supply values that underestimated medication compliance, particularly in long-term care (LTC). In this study we examined the impact of oral bisphosphonate exposure misclassification in days supply values on estimates of drug effectiveness in reducing hip fractures.
Methods: We used Ontario administrative claims data to identify new users of oral bisphosphonates aged 66 or more years between 2001 and 2011. Medication compliance was quantified by the proportion of days covered (PDC) and categorised into groups according to a 365-day ascertainment period. PDC was calculated using observed and cleaned days supply values. Hip fracture rates within 365 days after the ascertainment period were calculated using Cox proportional hazard models, adjusted for behavioural and fracture risk factors. Low compliance (PDC<20%) was the referent. Analyses were completed overall and separately for patients in community and LTC settings.
Results: The rate of hip fracture was higher in LTC (2.4/100 patient-years) than in the community (1.0/100 patient-years). Overall, cleaning days supply values increased the estimated benefit of high compliance (PDC≧80%) on fracture prevention (HRobserved=0.74, 95% CI=0.66-0.83; HRcleaned=0.65, 95% CI=0.57-0.74). Risk estimates were similar among community-dwelling patients (HRobserved=0.68, 95% CI=0.600.77; HRcleaned=0.65, 95% CI=0.560.75), yet differed substantially in LTC before data cleaning (HRobserved=0.96, 95% CI=0.731.26; HRcleaned=0.64, 95% CI=0.460.91).
Conclusion: Misclassified days supply values can bias estimates of drug effectiveness. Larger effects on risk estimates were noted in LTC, where fracture risk is highest. Overall, little change was noted in the community setting, where most studies are completed. Thus, results caution researchers to examine days supply accuracy in pharmacy claims data when estimating the relationship between drug exposure and health outcomes, particularly when including LTC settings.
Disclosure: The authors declared no competing interests.