Background  Little information is available regarding variations in diabetes mellitus (DM) outcomes by race at the level of individual physicians.

Methods  We identified 90 primary physicians caring for at least 5 white and 5 black adults with DM across 13 ambulatory sites and calculated rates of ideal control of hemoglobin A_1c (HbA_1c) (<7.0%), low-density lipoprotein cholesterol (LDL-C) (<100 mg/dL), and blood pressure (<130/80 mm Hg). We fitted hierarchical linear regression models to measure the contributions to racial disparities of patient sociodemographic factors, comorbidities, and physician effects. Physician effects modeled the extent to which black patients achieved lower control rates than white patients within the same physician’s panel (“within-physician” effect) vs the extent to which black patients were more likely than white patients to receive care from physicians achieving lower overall control rates (“between-physician” effect).

Results  White patients (N = 4556) were significantly more likely than black patients (N = 2258) to achieve control of HbA_1c (47% vs 39%), LDL-C (57% vs 45%), and blood pressure (30% vs 24%; P < .001 for all comparisons). Patient sociodemographic factors explained 13% to 38% of the racial differences in these measures, whereas within-physician effects accounted for 66% to 75% of the differences. Physician-level variation in disparities was not associated with either individual physicians’ overall performance or their number of black patients with DM.

Conclusions  Racial differences in DM outcomes are primarily related to patients’ characteristics and within-physician effects, wherein individual physicians achieve less favorable outcomes among their black patients than their white patients. Efforts to eliminate these disparities, including race-stratified performance reports and programs to enhance care for minority patients, should be addressed to all physicians.

Racial disparities in the quality of diabetes mellitus (DM) care and outcomes are well documented. Black patients with DM are less likely than white patients to receive recommended processes of care, including hemoglobin A_1c (HbA_1c) and lipid testing.^1,2 Ideal DM treatment goals, such as glycemic, cholesterol, and blood pressure (BP) control are also less commonly achieved among black patients compared with white patients.^3–8 Ultimately, black patients are more likely than white patients to experience poor long-term diabetic outcomes, including diabetic retinopathy,⁹ lower extremity amputations,^2,10,11 and chronic kidney disease.¹²

Identifying the underlying reasons and potential solutions for these differences in quality of care and outcomes is a high priority.^13–15 Although quality improvement programs can eliminate racial disparities in process measures of DM care, disparities in intermediate outcomes often persist,^6,7 highlighting the importance of monitoring outcomes of care stratified by race. In addition to an increased focus on outcome measures of care, location of care is an increasingly recognized mediator of some racial disparities.¹⁶ Among Medicare enrollees with DM in health plans, approximately two-thirds of racial differences in the control of glucose, cholesterol, and BP are explained by racial differences within health plans, whereas one-third are due to black enrollees receiving treatment in lower-performing health plans.¹⁷

Although prior studies^17–20 have focused on the role of hospitals, health plans, and regions as mediators of racial disparities, little is known about the role of variation among individual physicians. Population-level disparities may arise if black patients disproportionately receive care from physicians who provide lower quality DM care (between-physician effect) or if black patients receive lower quality care than white patients within the same physician’s panel (within-physician effect). In addition, features of individual physicians or their patients may predict more equal care for patients. For example, physicians who provide higher overall quality may provide more uniform care and thus be less likely to have large racial differences in care among their patients (ie, smaller within-physician effect). Physicians with a more diverse patient panel may be more comfortable caring for minority patients and thus have smaller racial differences in outcomes among their patients. The use of rigorous hierarchical models to evaluate physician-level effects on health disparities can serve as a model for other health care organizations seeking to understand racial differences in care. These analyses may not capture the full spectrum of explanatory factors related to differential outcomes within a physician’s panel, such as complex social and behavioral factors. As such, they should not be used to assign sole responsibility to an individual physician but rather to highlight patterns of variation amenable to focused intervention.

Therefore, our study had the following 2 main objectives: (1) to assess the extent to which racial disparities in intermediate outcomes of DM care are related to within-physician vs between-physician effects and (2) to determine whether overall quality or a more diverse patient panel are associated with decreased racial disparities within individual physicians’ patient panels.