Long-term reproducibility of tests of small airways function. Comparisons with spirometry

We used a tracking index to measure the reproducibility of single breath nitrogen test variables (CV/VC, CC/TLC, ...N 2 /L) and spirometric variables (FEV 1 and FEV 1 /FVC) and to compare the characteristics of individuals whose pulmonary function tracks well with those whose pulmonary function tracks poorly. Data were derived from two cohorts followed longitudinally over a 9-11 year period. All variables were adjusted for age, sex and height by expressing them as Z-scores. In all smoking groups and in both cohorts, the tracking index was highest for FEV 1 , indicating that this variable was the most reproducible over the period of follow-up; ...N./L and FEV 1 /FVC had very similar but slightly lower tracking indices; CV/VC consistently had the lowest tracking indices. Tracking indices were generally higher in smokers than in nonsmokers. The reproducibility of CC/TLC increased over the period of follow-up whereas the FEV 1 reproducibility remained constant. We found no significant difference between those with high tracking indices and those with low indices in terms of sex, smoking status, prevalence of respiratory symptoms, history of respiratory disease, and rate of decline of FEV 1 . We conclude that FEV 1 is the most reproducible of the variables examined, both within and between individuals, and that poor tracking or reproducibility are not related to smoking or to the presence of respiratory symptoms or disease.