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

The aim of this study was to characterise adherence in an adult population with cystic fibrosis (CF) and to investigate if variation in lung function was a predictor of adherence to treatment.The adherence of patients aged ≥16 years from an adult CF centre was measured by medication possession ratio (MPR) and self-report. Patients were assigned to one of three adherence categories (<50%, 50 to <80%, ≥80%) by their composite score (MPR). Ordinal regression was used to identify predictors of adherence, including coefficient variation measures for forced expiratory volume in 1 s (FEV1 ), weight and C-reactive protein concentration, measured from 6 months and 12 months before baseline.MPR data for 106 of 249 patients (mean age 29.8±9.2 years) was retrieved, indicating a mean adherence of 63%. The coefficient of variation for FEV1 was inversely related to adherence and was a univariate predictor of adherence (6 months: OR 0.92, 95% CI 0.87-0.98, p=0.005; 12 months: OR 0.94, 95% CI 0.93-0.99, p=0.03) and remained significant in the final models. The coefficient of variation of weight and C-reactive protein were not predictive of adherence.The coefficient of variation of FEV1 was identified as an objective predictor of adherence. Further evaluation of this potential marker of adherence is now required.