Replication of the Scandinavian Simvastatin Survival Study using a primary care medical record database prompted exploration of a new method to address unmeasured confounding

Purpose To examine whether identifiable study characteristics and/or analytic methods used determine observational study validity, as assessed by replicating randomized controlled trials using observational data. Methods A cohort from the United Kingdom General Practice Research Database (GPRD) was used to replicate the Scandinavian Simvastatin Survival Study RCT, which investigated statin treatment of hypercholesterolemic subjects with coronary heart disease. All aspects of the RCT except randomization were replicated to the extent possible in the GPRD study, which included 2,871 Unexposed and 1,280 statin-treated Exposed subjects. Results Overall mortality [adjusted hazard ratio 0.71 (0.53-0.96)] and myocardial infarction [adjusted HR 0.79 (0.61-1.02)] decreased in the GPRD study similar to the RCT. Coronary revascularization increased two-fold in the GPRD study, whereas it decreased significantly in the RCT [0.63 (0.54-0.74)]. This latter disparity prompted use of a new methodology to adjust for unmeasured confounding, which yielded an adjusted HR [1.0 (0.75-1.33)] more comparable to the RCT. Conclusions This study provides additional evidence that a replicated GPRD observational study can yield results reasonably similar to a RCT. More important, it provides preliminary evidence suggesting that a new analytic methodology may adjust for unmeasured confounding, the major limitation to research using observational data. Copyright (c) 2008 John Wiley & Sons, Ltd.