Accuracy of the days' supply and the number of refills allowed recorded in Québec prescription claims databases for inhaled corticosteroids

Objectives and hypotheses Adherence to inhaled corticosteroids (ICS) is a major issue in asthma. This study aimed to estimate the accuracy of the days’ supply and number of refills allowed, variables recorded in Quebec claims databases and used to estimate adherence, and to develop correction factors, if required. We hypothesised that the accuracy of the days’ supply for ICS would be low whereas the accuracy of the number of refills allowed would be high. Setting 40 community pharmacies in Quebec (Canada) and a medication registry. Participants We collected data for 1108 ICS original prescriptions stored in the 40 pharmacies (sample 1), and we obtained a second sample of 2676 ICS prescriptions selected from reMed, a medication registry (sample 2). Primary and secondary outcomes We estimated the concordance of the days’ supply and number of refills between Quebec claims databases and the original prescription from sample 1. We developed a correction factor for the days’ supply in sample 1 and validated it in sample 2. Analyses were stratified by age: 0–11 and 12–64 years. Results In sample 1, the concordance for the days’ supply was 39.6% (95% CI 37.6% to 41.6%) in those aged 0–11 years and 56% (54.9% to 57.2%) in those aged 12–64 years. The concordance increased to 59.4% (58.2% to 60.5%) in those aged 0–11 years and 74.2% (73.5% to 74.9%) in those aged 12–64 years after applying the correction factors in sample 2. The concordance for the refills allowed was 92.1% (91% to 93.1%) in those aged 0–11 years and 93.1% (92.5% to 93.7%) in those aged 12–64 years in sample 1. Conclusions The accuracy of the days’ supply was moderate among those aged 0–11 years and substantial among those aged 12–64 years after applying the correction factors. The accuracy of the number of refills was almost perfect in both groups.