Abstract
OBJECTIVES: To describe the short- and medium-term repeatability of lung clearance index (LCI(2.5) ) in infants and calculate the number of patients needed to enrol in a study (N) using LCI(2.5) as a primary outcome. METHODS: 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(2.5) in infants with cystic fibrosis (CF) or recurrent wheeze (RW) measured by the nitrogen multiple-breath washout test (N(2) -MBW).
Using these variability data, the N to reach 90% test power at the level of statistical significance (0.05) was calculated. RESULTS: Forty infants with CF and 21 with RW were enrolled.
Initial N(2) -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(2.5) 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(2.5) value.
In clinically stable patients, LCI(2.5) did not significantly change during the 8-month follow-up. Mean LCI(2.5) 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(2.5) and minimal difference to be detected would be 2.0. CONCLUSION: N(2) -MBW may be a reproducible tool with reasonable test power to detect differences in infant studies.
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