Objectives: To develop mean and 95% confidence limits for the lower limit of normal (LLN) values for forced expiratory volume in 3 s (FEV3)/FVC ratio for Latin, black, and white adults; to ascertain comparative variability of the FEV1/FVC ratio, the FEV3/FVC ratio, and forced expiratory flow, midexpiratory phase (FEF25–75) in never-smoking adults; to evaluate their utility in measuring the effect of smoking on airflow limitation; and to develop and use the fraction of the FVC that had not been expired during the first 3 s of the FVC (1 − FEV3/FVC) to identify the growing fraction of long-time-constant lung units.
Design: Analysis of the Third National Health and Nutrition Examination Survey (NHANES III) database of never-smokers and current smokers.
Participants: A total of 5,938 adult never-smokers and 3,570 current smokers from NHANES III with spirometric data meeting American Thoracic Society standards.
Measurements and results: After establishing new databases for never-smokers and current smokers, we quantified the mean and LLN values of FEV3/FVC in never-smokers, and identified spirometric abnormalities in current smokers. When associated with older age, FEV3/FVC decreases and 1 − FEV3/FVC increases as FEV1/FVC decreases. On average, using these measurements, the condition of current smokers worsened about 20 years faster than that of never-smokers by middle age. If < 80% of the mean predicted FEF25–75 was used to identify abnormality, over one quarter of all never-smokers would have been falsely identified as being abnormal. Using 95% confidence limits, 42% of 683 smokers with reduced FEV1/FVC and/or FEV3/FVC would have been judged as normal by FEF25–75.
Conclusions: FEV1/FVC, FEV3/FVC, and 1 − FEV3/FVC characterize expiratory obstruction well. In contrast, FEF25–75 measurements can be misleading and can cause an unacceptably large number of probable false-negative results and probable false-positive results.