A STRATEGY TO REDUCE HEALTHY WORKER EFFECT IN A CROSS-SECTIONAL STUDYOF ASTHMA AND METALWORKING FLUIDS

This report describes the reanalysis of a cross-sectional study of ast hma in a large cohort of autoworkers with exposure to metalworking flu ids (MWD). There is strong evidence from case reports, clinical studie s, and medical surveillance data that exposure to MWF can cause asthma , yet no association was found in the original analysis. The central h ypothesis of the reanalysis was that the absence of an association bet ween asthma and MWF exposure was the result of bias caused by the self -selection of asthmatics out of exposed jobs. We addressed the potenti al job transfer bias by redefining exposure and disease status at the time of asthma onset, rather than at the time of the health survey. Th is permitted us to treat the cross-sectional study as if it were a his torical cohort study, despite the fact that the population was a biase d sample of the full cohort. This approach resulted in a significantly elevated incidence rate ratio of 3.2 (95% Cl: 1.2-8.3) for synthetic MWF estimated in a Cox proportional hazards model. Although the cross- sectional design makes it impossible to document or control for differ ential selection out of the workforce, the approach described here pro vides a strategy for reducing the healthy-worker effect due to job tra nsfer bias in cross-sectional studies. (C) 1997 Wiley-Liss, Inc.