Objective: To investigate in children with cystic fibrosis (CF) and children without CF: (1) the test -retest reproducibility of a 20 min resting energy expenditure (REE) measurement; and (2) the long-term reproducibility of REE measurements in children with CF using longitudinal data. Design: Cross-sectional study and longitudinal cohort. Setting: A tertiary referral paediatric hospital. Subjects: A total of 31 (11 male, 20 female) children (aged 12.8 AE 3.6 y) with CF and 32 (14 male, 18 female) healthy children without CF (aged 12.2AE 2.3 y) were enrolled in the short-term reproducibility study. Long-term REE measurement reproducibility was assessed in another 14 children (5 male, 9 female) with CF, comparing their initial REE measurement with a subsequent measurement 1 -2 y later. Methods: All children had measurements of height, weight, skinfold thickness and indirect calorimetry. Results: There was no statistically significant difference in REE between repeated measurements in children with CF (mean AE s.d., 6240AE 1280 and 6220 AE 1315 kJ=24 h) and in the children without CF (6040 AE 956 and 6015 AE 943 kJ=24 h). For the children with CF, the intraclass correlation coefficient was 0.99 and for children without CF the intraclass correlation coefficient was 0.97. The measurement errors were 119 and 177 kJ, respectively. Approximately 80% of the variation in REE in the CF group and 70% in the group without CF was explained by fat-free mass (FFM). Analysis of the longitudinal CF data show there was no difference in REE between a child's first measurement (5140 AE 1140 kJ) and their subsequent measurement (5460 AE 1190 kJ), after adjustment for changes in body size between the measurements. Conclusion: This study has demonstrated that a short-term 20 min REE measurement is reproducible and therefore valid in children with CF and children without CF. These results also indicate that in children with CF, long-term REE measurements are reproducible.

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