IMPACT OF REFERRAL BIAS ON CLINICAL AND EPIDEMIOLOGIC STUDIES OF ALZHEIMERS-DISEASE

We used the resources of the Rochester Epidemiology Project to compare sociodemographic and clinical characteristics in three groups of Alzh eimer's disease patients. The first group included incidence cases occ urring among residents of Rochester, Minnesota (population-based serie s; n = 241). The second group was a sample of patients referred to the Mayo Clinic from the remainder of Minnesota and the four surrounding states (n = 58); the third was a sample referred from the remainder of the United States (n = 94). Patients from Rochester were more frequen tly women, less highly educated, less commonly white collar workers, m ore frequently institutionalized, less frequently married, and more of ten lived alone than those in the two referral groups; Patients from R ochester also had a more advanced age of onset of dementia. For occupa tion, education, and living arrangement, the differences across groups increased with increasing distance of referral. Clinical and epidemio logical studies based on patients referred from primary to secondary o r tertiary care centers may suffer from severe selection bias.