Identifying confounding by indication through blinded prospective review

Confounding by indication is a relentless threat to validity in observation al studies of treatment effects. Multivariable models allow adjustment for known and readily measurable prognostic factors, but they may incompletely or inaccurately represent the underlying overall perceived risk of treatmen t. To incorporate practitioners' judgments about treatment indication and p reprocedural prognosis into an observational study of cerebral aneurysm tre atments, the author and colleagues presented patient characteristics and ra diographic images from 179 aneurysm cases (University of California, San Fr ancisco, 1990-1997) to panels of practitioners who were blinded as to actua l treatment selection and outcome. In this way, the review process was desi gned to recreate the presentation of information in a prospective study. Ju dgments about inclusion and prognosis were reproducible. Perceived prognosi s correlated with complication rates and provided information not present i n a multivariable model including all available clinical characteristics. T he association between treatment modality and outcome was examined while st ratifying and adjusting for differences in perceived prognosis. Blinded pro spective review may provide an unbiased observational study design with whi ch to define a cohort that could have received any of the treatments being compared and to measure and adjust for overall perceived procedural risk.