Epidemiologic research in an integrated regional medical care system: the Marshfield Epidemiologic Study Area.

To capitalize on Marshfield Clinic's advantages for population-based health research, we developed the Marshfield Epidemiologic Study Area (MESA). Marshfield Clinic is an integrated system consisting of a large multispecialty clinic and 23 affiliated clinics. Clinic physicians provide virtually all of the medical care, both inpatient and outpatient, for residents of the area. MESA consists of 14 ZIP codes in which over 95% of the 50,000 residents and most significant health events are captured in Marshfield Clinic databases, including all deaths, 94% of hospital discharges, and 92% of medical outpatient visits. MESA exemplifies the research potential of integrated medical care systems and the efforts required to realize that potential. Because it is representative of a defined population and provides an unselected sample of patients, MESA is well suited for epidemiologic research and research elucidating the clinical spectrum and natural history of diseases and the effectiveness of treatment.