BIAS IN CASE-CONTROL STUDIES OF SCREENING EFFECTIVENESS

Screening programs, such as annual mammography, are undertaken to redu ce mortality and/or morbidity from chronic diseases such as cancer. Ma tched case-control studies have been used to assess the effectiveness of screening programs because of their relative simplicity and low cos t. In such studies, the exposure history for controls consists of the number of screening examinations received prior to the date of diagnos is of the matched case. The authors know of no methodological evaluati ons that demonstrate the validity of such case-control studies, To exa mine the possible existence of bias due to design rules, the authors d eveloped a simple deterministic model, which is used to calculate expe cted screening and disease patterns in a cohort. Cases and matched con trols are selected from the cohort, and their screening histories are used to calculate an odds ratio, as is commonly done in practice. Resu lts utilizing this simple model suggest that systematic inclusion of t he examination from which diagnosis is made, which is the approach typ ically used in practice, leads to a positive bias (odds ratio > 1) in the absence of any real effect. Systematic exclusion of this examinati on appears to lead to a negative bias (odds ratio < 1). Although this simple approach has several limitations, the results suggest that a co mmonly used method of conducting case-control studies may yield biased odds ratios. Possible methods to reduce this bias may exist, such as defining exposure intervals differently.