Background Lung function is an important indicator of cystic fibrosis disease status and those with better forced expiratory volume in 1 s (FEV₁)% predicted have tended to report a better health-related quality of life (HRQoL) in cross-sectional studies. The relationship between lung function and HRQoL over time is unknown. This work assesses the natural progression of HRQoL reporting over many years and compares assessments across a whole decade and evaluates the relationship between lung function and HRQoL longitudinally.

Methods Demographic (age, gender), clinical (FEV₁% predicted, body mass index, diabetes, Burkholderia cepacia complex, intravenous access device and nutritional status) and HRQoL (Cystic Fibrosis Quality of Life Questionnaire) variables were obtained every 2 years over a 12-year period (seven time points from 1998 to 2010).

Results HRQoL and lung function declined slowly over time and significant decade changes were observed for FEV₁% predicted and the nine domains of the Cystic Fibrosis Quality of Life Questionnaire. The results of random coefficient modelling indicated that, at the population level, decreasing FEV₁% predicted was associated with decreasing HRQoL after adjusting for confounding variables. However, the percentage of patients for whom a decrease in lung function was associated with a decrease in HRQoL differed according to the quality of life domain.

Conclusions HRQoL and FEV₁% predicted decline slowly; nevertheless, a decrease in lung function predicted a decrease in HRQoL over time.