Estimation of ventilatory capacity in subjects with unacceptable lung function tests.

Based on pulmonary function data collected annually for six years on 540 Vermont granite workers, FEV1 in survey 1 was estimated by extrapolating back from subsequent measurements. The extrapolation method was found to fit the observed data of subjects with reproducible initial values very well (R2 = 0.87). Extrapolated FEV1s for workers unable to perform an adequate pulmonary function test according to the standards of the American Thoracic Society were compared to extrapolated values in the rest of the cohort. After adjusting for confounding, subjects with test failure in survey 1 had a lower extrapolated FEV1 than the rest of the cohort (p = 0.07). The mean extrapolated FEV1 of the 71 workers with an initial test failure was only 95% of a predicted value derived from the group with reproducible data, and the per cent predicted decreased from 98% to 71% as the number of test failures in the follow-up surveys increased (p = 0.0004). The American Thoracic Society and the Epidemiology Standardization Project currently recommend that test failures be excluded from the analysis of epidemiological data. Our findings suggest that alternative strategies for handling non-reproducible lung function may need to be explored in order to avoid selection bias.