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Background Lung clearance index (LCI 2.5), measured by nitrogen-multiple breath washout (N2-MBW), is a sensitive measure of ventilatory inhomogeneity that can be performed awake/unsedated from aged 3 years. However, concerns have been raised about the feasibility of LCI2.5 measurement by N2-MBW in adults with cystic fibrosis (CF), especially those with advanced lung disease, due to prolonged test durations. We assessed the feasibility, technical acceptability and patient perception of N2-MBW in adults and children/adolescents with CF within the exercise as an airway clearance technique-CF feasibility trial.Methods N 2-MBW (Exhalyzer-D, EcoMedics, Switzerland) was performed on two separate occasions, by multidisciplinary trial staff. Data were centrally over-read (European Cystic Fibrosis Society LCI Core Facility). Outcomes included the proportion of technically acceptable tests, numbers of trials per test, times per trial and total time to perform N2-MBW. Comparisons were made according to age group and forced expiratory volume in 1 s (FEV1) category (>70% vs <70% predicted). Participants also completed a questionnaire assessing acceptability and willingness to repeat testing.Results 49 participants (21 female; age: 10–55 years, FEV 1: 38%–113% predicted) completed N2-MBW. Median (IQR) test durations were similar for adults (42 (27–55) min) and children (37 (26–44) min) with CF. Technically acceptable LCI2.5 results were obtained for 90/94 (95.8%) tests. Most participants rated the test easy to perform (87%) and comfortable (93%), with none reporting unwillingness to repeat N2-MBW testing.Conclusions N 2-MBW testing is feasible, technically acceptable and well-tolerated in people with CF across a wide age and disease severity spectrum. With appropriate standardised training and over-reading, LCI2.5 represents a robust outcome measure for consideration in CF clinical trials.Trial registration number NCT05482048.