One of the challenges of the national initiative, Healthy People 2010,1 is to support interventions that will reduce social disparities in health. While social disparities such as differences in education, income, race or ethnicity may affect health, the mechanisms are poorly understood. If social disparities in health originate in childhood, are current social disparities in health modifiable and are they the responsibility of a medical provider or health plan? Nonetheless, modifiable factors may exist at the individual, neighbourhood or system level that mediate (explain) social disparities in health and that may be suitable targets for interventions aiming to reduce disparities. Our aim was to survey and prospectively follow a large, diverse and well-characterized population with diabetes and to collect data on risk factors which may affect diabetes health outcomes but which may differ substantively in prevalence or effect size across ethnic groups or educational levels.
The 2002 Institute of Medicine report, ‘Unequal Treatment: Confronting Racial and Ethnic Disparities in Health Care’,2 detailed the socioeconomic fragmentation of health care quality and access, and its differential, negative impact on minorities. As access to health care is an important determinant of health outcomes and is in turn associated with ethnicity and socioeconomic position, it represents a potent source of confounding bias in population-based studies of social health disparities.
At the population level, identifying antecedents and determinants of social differences in disease progression remains a challenge. There are nationally representative surveys (e.g. National Health and Nutrition Examination Survey) that identify members of the general population with diabetes and that can estimate risk factors that are associated with diabetes complications, but these cohorts are often cross-sectional or panel studies and have limited ability to track continuous changes in individual-level health care, outcomes and risk factors over time. Longitudinal, population-based studies often have too few minority subjects with diabetes to reveal racial or ethnic differences.
The relatively uniform access to care in a managed care health plan such as Kaiser Permanente provides an advantageous setting in which to conduct a longitudinal study of social disparities in health and study findings may be compared with population-based studies, where health care access and quality may vary by socioeconomic position.
We established the Kaiser Permanente Northern California Diabetes Registry (‘Registry’) in 1993 using standardized criteria (Table 1) to identify and prospectively follow members with diabetes, to measure prevalence and incidence of diabetes and its co-morbidities, to understand factors associated with disease progression and complications, and to evaluate health care processes and outcomes. The Registry has an estimated sensitivity of 99% based on chart review validation (unpublished results). We conducted the first survey of the Registry in 1994–97 (Diabetes Registry Questionnaire) among all Registry members over 19 years of age. The primary goal of that survey was to capture individual-level information on the clinical characteristics of diabetes, age at diagnosis, ethnicity, education, health-related behaviours and diabetes family history. There were 77 726 respondents (83% response rate among eligible members) and that survey cohort has been the basis for numerous publications regarding the epidemiologic and health services aspects of diabetes.3–9
We previously reported findings regarding ethnic disparities in the incidence of myocardial infarction, stroke, congestive heart failure, end-stage renal disease and lower-extremity amputation among diabetic African American, Asian, Latino and Caucasian members of Kaiser Permanente Northern California (‘Kaiser’),7 a population with uniform access to care.10 Socioeconomic disparities in diabetic complications based on educational attainment and income have been reported in other populations.11,12
The National Institutes of Health provided funding to the Kaiser Division of Research and the University of California, San Francisco School of Medicine, to conduct the Diabetes Study of Northern California (DISTANCE). This study was approved by their respective Institutional Review Boards.