Comparison of flow-volume and closing volume variables in a random population

Data obtained from results of single-breath N2, or closing volume, tests were compared with maximal expiratory flow-volume parameters in 725 subjects, 25 to 54 years of age, from a random, stratified, cluster sample of the population of Tucson, Arizona. Results of the 2 tests showed poor concordance. A greater proportion of abnormalities was revealed by maximal expiratory flow-volume measurements than by the single-breath N2 test in population subgroups defined as other than "normal". Among the 114 subjects reporting physician-confirmed asthma, chronic bronchitis, or emphysema, less than one half of those with maximal expiratory flow-volume abnormalities also had an abnormality in the single-breath N2 test. Results of this comparison do not indicate that the closing volume test, when applied to a general population, consistently reveals abnormalities of clinical significance.