RationaleWhile lung clearance index (LCI) is a sensitive marker of small airway disease in individuals with cystic fibrosis (CF), less is known about longitudinal changes in LCI during routine clinical surveillance.ObjectivesTo describe the longitudinal course of LCI in children with CF during routine clinical surveillance and assess influencing factors.MethodsChildren with CF aged 3–18 years performed LCI measurements every 3 months as part of routine clinical care between 2011 and 2018. We recorded clinical data at every visit. We used a multilevel mixed-effect model to determine changes in LCI over time and identify clinical factors that influence LCI course.Measurements and Main ResultsWe collected LCI from 1204 visits (3603 trials) in 78 participants, of which 907 visits had acceptable LCI data. The average unadjusted increase in LCI for the entire population was 0.29 LCI units·year−1 (95% CI 0.20–0.38). The increase in LCI was more pronounced in adolescence, with 0.41 units·year−1 (95% CI 0.27–0.54). Colonisation with either Pseudomonas aeruginosa or Aspergillus fumigatus, pulmonary exacerbations, CF-related diabetes, and bronchopulmonary aspergillosis were associated with a higher increase in LCI over time. Adjusting for clinical risk factors reduced the increase in LCI over time to 0.24 LCI units·year−1 (95% CI 0.16–0.33).ConclusionLCI measured during routine clinical surveillance is associated with underlying disease progression in children with CF. An increased change in LCI over time should prompt further diagnostic intervention.

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