The lung clearance index (LCI) is more sensitive than spirometry in detecting abnormal lung function in children with cystic fibrosis. LCI is thought to be independent of age, but recent evidence suggests that the upper limit of normal is higher in infants and preschool children than in older subjects. This study examines whether LCI remains independent of body size throughout childhood.Multiple-breath washout data from healthy children and adolescents were collated from three centres using the mass spectrometer system and the inert gas sulfur hexafluoride. Reference equations for LCI and functional residual capacity (FRC) were constructed using the LMS (lambda-mu-sigma) method.Data were available from 497 subjects (2 weeks to 19 years of age) tested on 659 occasions. LCI was dependent on body size, decreasing in a nonlinear pattern as height increased. Changes were particularly marked in the first 5 years of life. Height, age and sex were all independent predictors of FRC. Minimal between-centre differences allowed unified reference equations to be developed.LCI is not independent of body size. Although a constant upper normal limit would suffice for cross-sectional clinical assessments from 6 years of age, appropriate reference equations are essential for accurate interpretation of results during early childhood. KEYWORDS: Children, functional residual capacity, lung clearance index, reference values, ventilation inhomogeneity M easuring the efficiency of ventilation distribution within the lung offers the exciting potential to detect early disease processes missed by conventional flow-based lung function techniques such as spirometry. Multiple-breath inert-gas washout (MBW) is a noninvasive tidal breathing test, feasible across all ages, which offers improved sensitivity, compared with spirometry, to detect early cystic fibrosis lung disease throughout childhood [1][2][3].It is generally thought that LCI is independent of body size. However, cross-sectional studies in infants have suggested that the upper limit of normal (ULN) for global measures of ventilation distribution inhomogeneity, such as lung clearance index (LCI), may be higher compared with older subjects [3,4]. Such differences, which could arise from developmental changes or differences in measurement conditions, may preclude identification of early lung disease in both cross-sectional and longitudinal studies. Furthermore, appropriate normative data for functional residual capacity (FRC) derived from MBW (FRCMBW) spanning the paediatric age range are lacking. The aim of this study was to investigate the relationship between body size and LCI from infancy to young adulthood, and establish reference equations for both LCI and FRCMBW across this age range.
METHODSMBW was performed in healthy children at three specialised paediatric centres in the UK, Sweden and Canada as described previously [3,[5][6][7][8][9][10]. Briefly, during wash-in, a dry air mixture containing Gustafsson) and protocols in all centres [11]. Please see the online su...