CAUTIONS IN THE USE OF ANTECEDENTS AS SURROGATES FOR CONFOUNDERS

When lacking information on confounding variables, epidemiologists hav e us surrogates which are antecedents of both the exposures and confou nders of interest. The usefulness of this strategy is explored in a se ries of scenarios for a prospective epidemiologic study wherein risk r atios relating antecedent to confounder, antecedent to exposure, and c onfounder to exposure were varied. Antecedent-adjusted, confounder-adj usted, and crude risk ratios were calculated and compared. The anteced ent-adjusted risk ratio was useful, that is, was closer to the confoun der-adjusted risk ratio than was the crude risk ratio, in 1,067 (49%) of 2,187 scenarios. The antecedent-adjusted risk ratio, the crude risk ratio, and the risk ratio relating confounder to exposure together pr edicted the usefulness of the antecedent (or any variable) as a confou nder proxy. The antecedent was useful in 97% of scenarios wherein: 1) the antecedent-adjusted risk ratio was less than the crude risk ratio, and the risk ratio relating confounder to exposure was greater than 1 .0, or 2) the antecedent-adjusted risk ratio was greater than the crud e risk ratio, and the risk ratio relating confounder to exposure was l ess than 1.0. In the remaining scenarios, it was useful only 5% of the time.