Abnormal preschool Lung Clearance Index (LCI) reflects clinical status and predicts lower spirometry later in childhood in cystic fibrosis.

•Preschool Commercial N2 based MBW informs on current and future CF clinical status•Preschool LCI relates to other current measures of concerning clinical status•Preschool LCI holds prognostic utility, informing on later spirometry outcomes.•Results strengthen the case for routine preschool MBW screening in CF subjects.