A method to calculate adherence to inhaled therapy that reflects the changes in clinical features of asthma

Rationale: Currently, studies on adherence to inhaled medications report average adherence over time. This measure does not account for variations in the interval between doses, nor for errors in inhaler use. Objectives: To investigate whether adherence calculated as a single area under the (concentration-time) curve (AUC) measure, incorporating the interval between doses and inhaler technique, was more reflective of patient outcomes than were current methods of assessing adherence. Methods: We attached a digital audio device (INhaler Compliance Assessment) to a dry powder inhaler. This recorded when the inhaler was used, and analysis of the audio data indicated if the inhaler had been used correctly. These aspects of inhaler use were combined to calculate adherence over time, as an AUC measure. Over a 3-month period, a cohort of patients with asthma was studied. Adherence to a twice-daily inhaler preventer therapy using this device and clinical measures were assessed. Measurements and Main Results: Recordings from 239 patients with severe asthma were analyzed. Average adherence that was based on the dose counter was 84.4%, whereas the ratio of expected to observed accumulated AUC,actual adherence, was 61.8% (P<0.01). Of all the adherence measures, only adherence calculated as AUC reflected changes in asthma quality of life, b-agonist reliever use, and peak expiratory flow over the 3 months (P<0.05 compared with other measures of adherence). Conclusions: Adherence that incorporates the interval between doses and inhaler technique, and calculated as AUC, is more reflective of changes in quality of life and lung function than are the currently used measures of adherence.