Background:
Knowledge of between-session variability of nitrogen multiple-breath washout (N2MBW) indices is crucial when designing longitudinal interventional studies and in disease monitoring using N2MBW as end-point. Such information is currently sparse.

Methods:
Monthly triplets of N2MBW were prospectively obtained from 14 children with CF during one year. Linear mixed models were used to analyze variability. Our aim was to assess between-session variability of N2MBW indices from repeated measurements and compare LCI derived from different software packages currently in use (TestPoint® vs. Spiroware®).

Results:
Baseline LCI (median; range) was 9.37 (6.82; 12.08). Between-session differences in LCI measurements were up to 25%. Intra Class Correlation-Coefficient was 0.82. There was no systematic difference between LCI measurements derived from the two software packages (p = 0.18); however, variability was significantly higher using Spiroware®.

Conclusions:
We report between-session variability of LCI using N2MBW in school-age children and adolescents with CF. LCI changes exceeding 25% may be considered clinically relevant. TestPoint® and Spiroware® can be used interchangeably in longitudinal studies.