Short and long term variability of indices from the single and multiple breath nitrogen test

We have studied the short and long term variabilities of indices derived from the single and multiple breath nitrogen tests in adolescents aged 12-18 years. In 333 boys and girls, up to 10 measurements were performed at half-year intervals; in 39 of them four measurements were also made at 10 min intervals. The standard deviations of the residual volume and the height of phase IV were roughly proportional to the magnitude of these indices, and so were best expressed by coefficients of variation, of which the means were 8% and 37% respectively. The measurement error in residual volume (RV) was small (1.4 to 1.9%) when compared to 'real' biological variability (8.9%). The variabilities of other tests were not correlated with their magnitude. Using autocorrelation coefficients, short term variability could be approximated from longitudinal data for the vital capacity (VC) and RV, but less well for other indices. The between-to-within variance ratios were rather low in the 39 healthy pupils. Based on these findings, RV and VC appear to be indices with a potentially good predictive value for lung volume assessment or for detecting abnormal lung growth in longitudinal studies of adolescents.