To describe the short- and medium-term repeatability of lung clearance index (LCI ) in infants and calculate the number of patients needed to enrol in a study (N) using LCI as a primary outcome.
An 8-month follow-up observational study was employed for assessing short-term [coefficient of repeatability (CR) and intraclass correlation (ICC)] and medium-term repeatability (Bland-Altman method) of LCI in infants with cystic fibrosis (CF) or recurrent wheeze (RW) measured by the nitrogen multiple-breath washout test (N -MBW). Using these variability data, the N to reach 90% test power at the level of statistical significance (0.05) was calculated.
Forty infants with CF and 21 with RW were enrolled. Initial N -MBW testing was successful in 33 and 17 patients, respectively. Follow-up data were available for 23 and 11 infants, respectively. Short-term repeatability of LCI was high (CR = 1.10 and 1.04 in CF and RW patients, respectively; ICC = 0.88 and 0.83 in CF and RW patients, respectively). The between-subject standard deviation was <13% of the actual LCI value. In clinically stable patients, LCI did not significantly change during the 8-month follow-up. Mean LCI change was -0.08 (1% of baseline) in CF and -0.05 (0.6%) in RW, with 95% limits of agreement being (-1.70; 1.53) in CF and (-1.51; 1.40) in RW patients. N = 23 infants if both intra-group differences of LCI and minimal difference to be detected would be 2.0.
N -MBW may be a reproducible tool with reasonable test power to detect differences in infant studies. This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.