Variation in lung function as a marker of adherence to oral and inhaled medication in cystic fibrosis

Study aim: The aim of this study was to characterise adherence in an adult population with CF and to investigate if variation in lung function was a predictor of adherence to treatment. Patients and methods: Patients aged ≥ 16 years from an adult CF centre undertook adherence measures by medication possession ratio (MPR) and self-report and were assigned to one of three adherence categories (<50%, 50-<80%, 80% and above) by their composite score (MPR). Ordinal regression was used to identify predictors of adherence including coefficient variation measures for forced expiratory volume in 1 second (FEV1), weight and C-reactive protein, measured up to 6 and 12 months. Results: MPR data for 106 of 249 patients [mean age 29.8 (±9.2) years] was retrieved, indicating a mean adherence of 63%. Coefficient of variation FEV1 was inversely related to adherence and was a univariate predictor of adherence (6 months: 0.92 [0.87-0.98] p= 0.005 and 12 months: 0.94 [0.93-0.99], p=0.03]) and remained significant in the final models. The coefficient variation of weight and C-reactive protein were not predictive of adherence. Conclusions: Coefficient of variation FEV1 was identified as an objective predictor of adherence. Further evaluation of this potential marker of adherence is now required.