Comparing Multiple Breath Washout Parameters in a Matched Cohort of Children With Cystic Fibrosis and Primary Ciliary Dyskinesia

Background Cystic Fibrosis (CF) and primary ciliary dyskinesia (PCD) are congenital disorders characterized by impaired mucociliary clearance, resulting in chronic airway infection, inflammation, and progressive lung damage. CF lung disease involves defective CFTR function throughout the entire airway epithelium, whereas PCD originates from dysfunction of motile respiratory cilia located predominantly in proximal airways. Diagnostic tools such as FEV 1 lack sensitivity to detect early peripheral lung damage. Lung clearance index (LCI), obtained via multiple breath washout (MBW), has emerged as a promising marker for early detection of lung abnormalities. Objectives To compare LCI between age and FEV 1 ‐matched people with (pw) CF and PCD (without highly effective CFTR modulator treatment), hypothesizing more abnormal LCI in CF due to more peripheral airway involvement. Secondary objectives included comparing phase III slope parameters (S cond x VT, S acin x VT) to explore ventilation inhomogeneity and localization of lung disease. Methods This retrospective cross‐sectional study included pwCF and pwPCD aged 6–20 years with available MBW and spirometry data (2011–2024). Results PwCF (n23, median age 14.4 y) had similar FEV 1 z‐scores compared to 23 pwPCD (median age 14.4 y) (−0.1 vs. –1.1, p: 0.33). LCI values were more abnormal in the CF group compared to the PCD group (7.8 vs. 7.1; p: 0.04). FEF 25‐75 %pred and phase III slope parameters were not significantly different, though a trend toward higher S cond x VT in CF was noted (0.08 vs. 0.06; p: 0.11). Conclusion Despite similar FEV 1 abnormalities, reflecting proximal airway impairment, pwCF exhibit greater peripheral airway involvement than pwPCD, as indicated by higher LCI values.