UCLA

Background: Spirometric values of 5880 never-smoking black, Latin, and white men and women in the Third National Health and Nutrition Examination Survey (NHANES-3) reference population were reviewed. Good published equations for forced expiratory volume in 1 second (FEV₁) over forced expiratory volume in 6 seconds (FEV₆) and FEV₁over forced vital capacity (FVC) often significantly mis-identified the lower limit of normal (LLN) targets in both younger and older adults. To improve detection of smaller airways disease in adults, we wished to redefine the LLN for these ratios and develop new ones for forced expiratory volume in 3 seconds (FEV₃)/FEV₆ and FEV₃/FVC. Methods: In each of 6 ethnic/gender, never-smoking NHANES-3 groups, arranged sequentially by age from 20.0 to 79.9 years, the values of FEV₁/FEV₆, FEV₁/FVC, FEV₃/FEV₆, and FEV₃/FVC were placed in groups of 40 so that the actual lowest second (5%) ratios could be identified. The slopes and intercepts of the resulting 24 linear equations through these lowest 5% ratios were then each adjusted by multiple iterations to best identify equations which actually identified the lowest 5% in both younger and older adults. Results: In all never-smokers, the new equations were closer to the 5% LLN targets than were those of Hankinson for FEV₁/FEV₆ and FEV₁/FVC and Quanjer for FEV₁/FVC. In 3508 NHANES-3 current smokers, the FEV₃/FEV₆ and FEV₃/FVC identified significantly more values below LLN than the FEV₁/FEV₆ and FEV₁/FVC. Conclusion: New simple linear iterative equations for FEV₁/FEV₆, FEV_1/FVC, FEV₃/FEV₆, and FEV₃/FVC to identify LLN are offered. None require exponents or logarithms. The latter 2 detected more abnormalities in current-smokers and likely better identify small airways disease in adults.