Background: Common measures of adherence to prescribed medications derived from administrative databases reflect both patients' and physicians' behavior, even if the measures are often interpreted as reflecting only the patient's adherence. Adherence to inhaled corticosteroids (ICSs) has been shown to be low among patients with asthma.
Objective: To develop a new measure of patients' adherence adjusted for prescription patterns and to evaluate the extent to which the low use of ICSs in asthma is due to patients' nonadherence or suboptimal prescribing practices.
Methods: The new measure of adherence, called the proportion of prescribed days covered (PPDC), is defined as the ratio of the total days' supply dispensed to the total days' supply prescribed during the study period. The PPDC is a modification of an existing adherence measure, the proportion of days covered (PDC). The PPDC and PDC for ICSs, therapy that should be prescribed for chronic daily use to patients with persistent asthma, were compared within a cohort of 4190 ICS-naïve patients with asthma aged 18-45 years derived from the administrative health databases of Quebec, Canada. We estimated the mean and the 95% confidence interval of the PPDC and PDC for ICSs over 1 year, and we calculated the part of nonadherence attributed to patients when measured with the PDC that can be attributed to nonoptimal prescribing of ICSs for chronic daily use with the following formula: [(1-PDC)-(1-PPDC)]/(1-PDC).
Results: The mean PPDC and PDC during the 1-year study were 52.6% (95% CI 51.6 to 53.6) and 19.1% (95% CI 18.6 to 19.6), respectively. Forty-one percent of nonadherence attributed to patients when measured with the PDC could be, in fact, attributed to nonprescribing of ICSs for chronic daily use.
Conclusions: Our new adherence measure, the PPDC, may be considered as another way to assess patient adherence, taking into account differing prescribing patterns.