Defined daily dose definition in medication adherence assessment in asthma

Adherence to inhaled corticosteroids (ICS) has been described as poor. In adherence studies, if the actual prescribed dosing is not available, generic defined daily doses (DDD) are applied instead when assessing adherence. We evaluated asthma patients' adherence in a large prospective follow-up survey. We also analysed whether World Health Organization (WHO) and Global Initiative for Asthma (GINA) reference doses give different results. The current study was cross-sectional and included respondents attending to HeSSup follow-up questionnaire in 2012. Altogether 1,141 of 12,854 adult participants answered positively to the question about having asthma. According to the Finnish Social Insurance Institutions' medication register, 686 of them had purchased ICS medication during 2011. DDDs for ICS by WHO as well as medium doses from GINA report were used as reference doses to evaluate adherence. To estimate adherence to ICS, the proportion of days covered (PDC) over one year was calculated for every patient. If the lower limit of GINA medium ICS dose was used as a reference, 65% of the patients were adherent (PDC ≥ 80%). Use of WHO's DDD as reference halved the proportion of adherent patients. Adherence was higher among those using a combination inhaler of corticosteroid and long-acting β₂-agonist compared to those using steroid only inhalers. Use of WHO's daily defined doses as reference values may lead to underestimation of adherence to inhaled corticosteroids. Thus, attention should be paid when choosing the reference doses for the evaluation of adherence to inhaled corticosteroids in asthma.