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, ΔN₂/L) and spirometric variables (FEV₁ and FEV₁/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 FEV₁, indicating that this variable was the most reproducible over the period of follow-up; ΔN₂/L and FEV₁/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 FEV₁ 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 FEV₁. We conclude that FEV₁ 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 be presence of respiratory symptoms or disease.