Spirometric "abnormality" in a normal male reference population: further analysis of the 1971 Oregon survey.

Conventional criteria for spirometric impairment (FVC and FEV1 less than or equal to 79% predicted, FEF25-75 less than or equal to 74% predicted, FEV1/FVC less than or equal to 0.69 for most ages) were applied to 507 healthy nonsmoking male subjects in the 1971 Oregon survey. Frequency of air flow impairment was higher than expected for a normal population. Of all subjects, 17.8% had an "abnormal" FEF25-75% and 19.7% an "abnormal" FEV1/FVC. "Decreased" lung volumes were seen in older subjects; of those over 55 years of age, 8.8% had a reduced FVC and 11.8% a reduced FEV1. These rates in a normal population may be used as a rough baseline for making inferences about a population under study. Similar rates were found in a cross-sectional survey of nonsmoking males representative of the population of a large industrial state. The major reason for these high rates of "abnormal" spirometry lies in the conventional definitions of abnormality. Tables showing cut-off points for abnormal spirometric values based on a 95% confidence interval for adult males of all ages and heights permit an alternative method for classification as abnormal.