Cystic Fibrosis (CF) lung disease is characterized by a marked heterogeneity. Sweat chloride‐level is a functional marker of the CF Transmembrane Regulator (CFTR) protein and could be an important predictor of later disease severity.

In this retrospective analysis children from the Rotterdam CF clinic with available sweat chloride level at diagnosis and at least one routine spirometry‐controlled volumetric chest CT scan in follow‐up were included. CT scans were scored using the CF‐CT scoring system (% of maximum). Associations between sweat chloride‐levels and CF‐CT scores were calculated using linear regression models, adjusting for age at sweat test and age at follow‐up. Because structural lung damage develops over the course of many years, effect modification by the age at follow‐up CT‐scan was tested for by age‐stratification.

In 59 children (30 male) sweat chloride was measured at diagnosis (median age 0.5 years, range 0‐13) and later chest CT performed (median age 14 years, range 6‐18). Sweat chloride was associated with significantly higher CT‐CT total score, bronchiectasis score, and mucus plugging score. Stratification for age at follow‐up in tertiles showed this association remained only in the oldest age group (range 15‐18 years). In that subgroup associations were found with all but one of the CF‐CT subscores, as well as with all tested lung functions parameters.

Sweat chloride‐level is a significant predictor of CF lung disease severity as determined by chest CT and lung function. This association could only be demonstrated in children with follow‐up to age 15 years and above.