Recent improvements in cystic fibrosis management have enabled an important subset of patients to maintain normal spirometry. Nevertheless, even in these patients with normal spirometry, chest computed tomography (CT) might identify structural lung abnormalities such as bronchiectasis, mucus plugging, bronchiolitis and air trapping.

Lung clearance index (LCI) has been shown to correlate well with structural pulmonary alterations seen on CT even in CF patients with well-preserved spirometry. In cystic fibrosis, a high LCI is associated with a worse feeling of illness assessed by the Cystic Fibrosis Questionnaire-Revised (CFQ-R). School-aged CF patients with normal spirometry also have normal aerobic function as assessed by peak oxygen uptake (V̇O2) measured during symptom-limited incremental cardiopulmonary exercise test. However, the ability of LCI to predict ventilatory abnormalities appearing at exercise in CF patients has not been investigated. The investigators therefore aimed to compare physiological parameters at exercise between CF patients with elevated LCI (i.e., LCI above the upper limit of normal [ULN]) and patients with normal LCI, all with preserved spirometry.

Inclusion Criteria:

• children between 10 to 17 years old with a diagnosis of cystic fibrosis confirmed by sweat test and genetic test
• pulmonary function test and incremental cardiopulmonary exercise test included in their follow-up
• lung function preserved (FEV1 greater than the lower limit of normal)
• affiliation to the French social security
• children and their legal guardians informed of the use of anonymised data

Exclusion Criteria:

• recent respiratory infection
• association with another obstructive respiratory disease, especially with asthma

Children between 10 to 17 years old with a diagnosis of cystic fibrosis