Objectives: To investigate the relationship between lung function and exercise capacity in adults with cystic fibrosis (CF), and to develop a CF-specific equation to predict Modified Shuttle Test (MST) performance from baseline data. Design: Cross-sectional, retrospective study. Setting: Adult CF centre. Participants: One hundred and twenty-seven patients with CF [61 male; mean age 25 years (range 17 to 52 years), mean forced expiratory volume in 1 second (FEV1) 56% predicted (range 15 to 124%)]. Main outcome measures: MST and FEV1. Results: Overall, a moderate-to-good relationship was found between lung function and MST performance (walking distance vs FEV1% predicted: r = 0.64, P = 0.01). This relationship between FEV1 and MST shows an obvious threshold at an FEV1 of 67% predicted. Above this threshold, no significant association was observed between FEV1 and MST performance. However, a strong relationship (MST vs FEV1% predicted: r ≥ 0.74, P <0.01 for men and r = 0.79, P <0.01 for women) was found below an FEV1 of 67% predicted. Conclusions: This study suggests that a strong association exists between lung function (FEV1% predicted) and MST (walking distance) in adult patients with moderate-to-severe CF (FEV1 <67% predicted). A reference equation for MST performance was developed for those patients with FEV1 ≤67% predicted, providing a tool to make an a-priori prediction of MST walking distance
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