Introduction: Multiple-breath washout (MBW) has been shown to be a sensitive outcome measure to detect lung function impairment and response to therapy in patients with cystic fibrosis (CF), but MBW data in patients with bronchiectasis of other etiologies remain limited.

Methods: To determine the feasibility and sensitivity of MBW versus spirometry to detect lung function impairment in children and adults with bronchiectasis, we performed both measurements in 231 patients with bronchiectasis (age range: 0.2 ? 79.3 years), including 128 patients with CF, 60 with primary ciliary dyskinesia, 33 with non-CF-bronchiectasis, 10 with postinfectious bronchiolitis obliterans and, in addition, 168 healthy controls and 76 patients with asthma as disease controls.

Results: Success rate of MBW in all 475 participants was 96.0 %. The FEV1 z-score was reduced (p < 0.0001) and the lung clearance index (LCI2.5) was elevated (p < 0.0001) in patients with bronchiectasis compared to all other groups. Overall groups LCI2.5 correlated strongly with FEV1 z-score (r = - 0.742, p < 0.0001). 33 % of patients with bronchiectasis had a normal FEV1 z-score but elevated LCI2.5 versus 2 % of patients that had a reduced FEV1 z-score but normal LCI2.5 (p < 0.0001). LCI2.5 was a superior parameter to detect lung function impairment in bronchiectasis than FEV1 z-score (AUC: 0.913 vs. 0.778).

Conclusion: Our data show that MBW is feasible and more sensitive than spirometry to detect lung function impairment in children and adults with bronchiectasis. This supports further development of LCI2.5 for diagnostic monitoring and as outcome measure in clinical trials.