Comorbidity and confounding in end-stage renal disease.

In the epidemiologic study of end-stage renal disease (ESRD), accurate adjustment for comorbidity is essential to controlling bias in the comparison of outcomes across groups of patients or dialysis facilities. Liu et al. derive and validate a novel comorbidity index using data from the United States Renal Data System. This novel index represents an important tool for observational research in ESRD but will not by itself overcome the potential problems of residual and unmeasured confounding in observational research.