Study Setting and Design

A population-based, case-control study was designed. This study was reviewed and approved by the Mayo Clinic and Olmsted Medical Center Institutional Review Boards and was performed according to the Declaration of Helsinki. All participants were identified by using the resources of the Rochester Epidemiology Project, a medical records linkage system that allows a user to search medical records, medical diagnoses, surgical interventions, and demographic information of virtually all persons residing in Olmsted County, Minnesota (2010 total county population = 144,248).^21-23 The characteristics of the Olmsted County population are similar to those of Minnesota and the Upper Midwest, although compared to the entire U.S., Olmsted County is less ethnically diverse, more educated, and wealthier.²¹ Nearly all health care to county residents is provided by Mayo Clinic and Olmsted Medical Center and their affiliated hospitals, and the Rochester Family Medicine Clinic. Only a small proportion of the population (approximately 2%) does not allow any of their medical records to be used for research.²¹

The Rochester Epidemiology Project enumerates the population by linking medical records across different health care providers to create a unique list of subjects with specified medical conditions and identification of residency status at each visit. This system serves as a data-retrieval system for a complete description of virtually all sources of medical and surgical care used by the Olmsted County population during a specified time. ^21-23