Spirometric “abnormality” in a normal male reference population: Further analysis of the 1971 oregon survey

Conventional criteria for spirometric impairment (FVC and FEV₁ ⩽ 79% predicted, FEF_25‐75 ⩽ 74% predicted, FEV₁/FVC ⩽ 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” FEF_25‐75% 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.