Seasonal fluctuation of lung function in cystic fibrosis: A national register-based study in two northern European populations

BACKGROUND: Many risk factors for lung disease in cystic fibrosis (CF) display a seasonal pattern yet it is unclear whether this is reflected in seasonal fluctuations in lung function. METHODS: We conducted a longitudinal study using CF registries in Denmark and the UK. 471 individuals with a median of 104 FEV(1) measurements per person and 7586 individuals with a median of nine FEV(1) measures per person were included from Denmark and the UK respectively. We estimated the effect of seasonality on percent predicted FEV(1) trajectories using mixed effects models whilst adjusting for clinically important covariates. RESULTS: We found no significant cyclical seasonal variation in lung function in either country. The maximum variation in percent predicted FEV(1) around the yearly average was estimated to be 0.1 percentage points (95%CI 0 to 0.21) and 0.14 percentage points (95%CI 0 to 0.29) in Denmark and the UK, respectively. When considering possible step-like changes between the four seasons, we found that lung function was higher in spring compared to winter in the UK (0.34 percentage points, 95%CI 0.1 to 0.59) though the difference was not of clinical significance. CONCLUSION: In both the UK and Denmark there may be small seasonal changes in lung function but this effect is not of clinical importance.