Age and height dependence of lung clearance index and functional residual capacity.

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.