Validation of Health Event Capture in the Marshfield Epidemiologic Study Area

For over 20 years, the Marshfield Epidemiologic Study Area (MESA) has facilitated population-based health research for the Marshfield Clinic (MC) and collaborating partners.1,2 MESA is a geographic area in central and northern Wisconsin, USA defined by ZIP codes. It is divided into two regions: MESA Central includes 14 ZIP codes surrounding Marshfield, and MESA North includes 10 ZIP codes surrounding MC regional centers in Ladysmith and Park Falls (figure 1). These MESA ZIP codes represent a subset of the MC primary service area where a high proportion of health care utilization among residents has historically been captured in the MC electronic health record (EHR).1–3 Figure 1 Marshfield Epidemiologic Study Area (MESA) Central and North regions within the Marshfield Clinic (MC) primary service area in Wisconsin, USA. MESA is unique in that it is a predominantly rural population-based cohort based on U.S. Census designations, and it is within an area that has been traditionally medically underserved and economically disadvantaged compared to urban or suburban counterparts.4 MESA represents an entire geographic population; in contrast to, for example, a typical managed care population that is defined by insurance type or employment status. In addition, through provision of primary, secondary, and tertiary care, the full clinical spectrum of disease is represented, not only the more severe cases often studied at hospital or referral-based research centers. MESA has been used in a variety of studies on disease incidence, prevalence, risk factors, and clinical outcomes. From its inception, MESA was designed to support medical effectiveness and outcomes research, making it particularly well suited for comparative effectiveness research.5 To date, there have been over 75 published studies addressing a broad diversity of health issues. In the past 10 years, MESA was instrumental in the establishment of the MC Personalized Medicine Research Project personalized medicine cohort and biobank and the Wisconsin Rural Women’s Health Study cohort.6,7 MESA was essential in supporting the Centers for Disease Control and Prevention (CDC) sponsored annual studies of seasonal influenza vaccines effectiveness and influenza surveillance.8,9 In addition, MESA was the basis for rural population epidemiologic studies of Barrett’s esophagus, asthma in farm children, systemic lupus, and cardiovascular disease, among other health conditions.10–13 MESA differs from other established epidemiologic cohorts in several respects. Though similar in concept and size to the Mayo Clinic Rochester Epidemiology Project (REP), REP represents the population of Olmsted County Minnesota and is largely an urban cohort with less than 20% of members from rural areas.14 The Survey of the Health of Wisconsin (SHOW) began in 2008 and, modelled after the National Health and Nutrition Examination Survey (NHANES), samples over 3,000 Wisconsin adults annually.15,16 SHOW does not have a link to medical records/care data. Other large population-based research cohorts (eg, Framingham Heart Study) tend not to link with medical records and may be limited only to specialized disease states.17 The MESA cohort is a cumulative residency database designed to identify and track residents in MESA ZIP codes since its establishment in 1991. Membership in this cohort is updated daily based on automated review of MC computerized data systems, thus requiring members to have both an address within a MESA ZIP code and activity within the MC system. It is comprised of current MESA cohort members as well as past MESA cohort members who have moved outside of MESA. Although they remain in the MESA cohort, the follow-up time for the latter group is terminated when they die or move out of MESA. Researchers can link MESA residency with the MC EHR. The MC EHR is an extensive archive of clinical encounters, diagnoses, procedures, medications, and laboratory results from more than 50 MC patient care locations throughout central, northern, and western Wisconsin communities. The utility of MESA as a resource for population-based health research depends heavily on the degree to which health events are captured in the MC EHR among the region’s residents. This study was conducted to determine the current level of inpatient and outpatient MC health event capture among people residing in MESA on February 22, 2011 according to MC records (referred to in this paper as ‘current MESA cohort members’). Specifically, we aimed to estimate: (1) the proportion of current MESA cohort members whose last outpatient encounter was at a MC facility, (2) how often MC locations are used by current MESA cohort members for outpatient health care, and (3) the proportion of current MESA cohort members whose most recent overnight hospital stay was at a MC affiliated hospital.