Pediatric Pulmonology 8:172-177 (1990) Tracking of Lung Function in Healthy Children and Adolescents Marienne zyxwvu E. Hibbert, M zyxwvut App Sci, Irene L. Hudson, PhD, Anna Lanigan, SRN, Louis 1. Landau, MD, and Peter D. Phelan, MD Summary. zyxwvutsrq Two hundred twenty-six healthy school chlildren, with a mean age of 8.8 years; 62 girls mean age 8.8, 48 boys mean age 12.6 and 51 girls mean age 12.6 years at the start, were enrolled in a longitudinal study zyxwvutsr of lung function and tested annually for 5 years. All were free of respiratory symptoms, and none smoked more than five cigarettes per week during the 5 years. Static and dynamic lung volumes (other than residual volume), maximum expiratory and inspira- tory flows, and maximum mouth pressures “track,” that is, individuals remain at a constant deviation from the sample mean over time. The data indicate that these measurements of lung function in healthy individuals grow in constant proportion relative to other healthy children and adolescents. Pediatr Pulmonol 1990; 8:172-177. Key words: Static and dynamic lung volumes; maximum flows; maximum mouth pres- sures; five annual testings in two cohorts; standard error of Tau; Jackknife procedure; Kendall’s coefficient of concordance. INTRODUCTION zyxwvutsrq Longitudinal data on lung function in childhood through adolescence to adult life are important to allow more accurate examination of an individual’s growth with respect to that of the whole cohort. ’,’ Such data will help us to understand better the growth of normal lung function over these phases of life. Few longitudinal studies of lung function in children and adolescents exist, and of those reported,3p10includ- ing the one using currently recommended standard test- ing techniques,”.’* none has studied healthy subjects throughout childhood and adolescence. There is a lack of general agreement on the best method for analyzing such longitudinal data. l3-I5 Few comparisons have been made of cross-sectional and lon- gitudinal studies in children and adolescents. In a study of adolescents, the longitudinal slope of the best-fitting model was greater than was the cross-sectionally derived model,16 and a recent study in children found the re- verse, but the differences diminished with non-linear models. I” However, determining longitudinal slopes by averaging individual regression lines may be unduly simplistic. l7 Tracking is a method that monitors whether for a growth variable of an individual the relative devi- ation from the population mean remains unchanged over time, hence, whether the parameter grows in constant proportion to the population mean. ’’ Tracking can also be thought of as a measure of whether the individual growth curves remain essentially of the same shape and do not cross one another.’’ Other tracking measures use the mean population curve and individual growth curves 0 1990 Wiley-Liss, Inc. to predict future values based on initial values and slope.” The tracking index technique’’ has been found satisfactory as a monitor of the growth of various anthro- pomorphic characteristics; such analysis is only possible with longitudinal data. A single report of a study can be found using McMa- han’s tracking index for following growth in vital capac- ity (VC) and timed forced expiratory volume (FEV,) over zyxwvu 5 years in a group of children randomly selected from a normal population at 6 years of age.4 This was found to be satisfactory over this limited age group. However, the study did not follow the cohort through adolescence into adult life. Furthermore, as this was a community-based study, children with lung disease were included. In the present study, we have examined tracking of a number of pulmonary function variables in two cohorts of healthy children and adolescents using McMahan’s tracking index, Tau, I’ and Kendall’s coefficient of con- From the Professorial Department of Thoracic Medicine, Biostatistics Unit, Royal Children’s Hospital, and Department of Paediatrics, Uni- versity of Melbourne, Melbourne, Australia. Received April 10, 1989; (revision) accepted for publication Novem- ber 18, 1989. This paper was supported by a grant from the National Health and Medical Research Council. Address correspondence and reprint requests to M. Hibbert, Profes- sorial Department of Thoracic Medicine, Royal Children’s Hospital, Flemington Road, Parkville, 3052, Victoria, Australia.