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

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.