This meta-analysis shows that African Americans have elevated A1C compared with non-Hispanic whites. The effect size estimate for this difference translates into an ~0.65% difference (pooled SD for A1C across studies of 2.1) in A1C levels between African Americans and non-Hispanic whites. Our findings confirm those of Trivedi et al. (36
), who, in a recent analysis of 1.8 million individual-level observations from 183 health plans, reported disparities between African Americans and non-Hispanic whites in glucose control.
Poor glycemic control is an important risk factor for diabetes complications, particularly microvascular complications (10
). The difference between racial groups found in this meta-analysis represents a potentially significant increase in the micro- and macrovascular complications associated with diabetes. The U.K. Prospective Diabetes Study found a 21% reduction in any outcome for every 1% reduction in A1C (37
). Thus, the difference found in this meta-analysis represents an ~15% reduction in risk for vascular complications among non-Hispanic whites. However, this does not explain the magnitude of difference in diabetes complications between African Americans and non-Hispanic whites. Although A1C control among African Americans likely contributes to their elevated risk of complications, it accounts for only a portion of their excess risk. The need to control A1C, blood pressure, and cholesterol risk factors among patients with diabetes has been previously reviewed (19
The consistency of the findings is no-table: 10 of 11 studies showed significance such that the range of effect sizes did not include zero. Furthermore, all analyses provided the same general results and therefore strongly support the conclusion that there are significant differences in A1C between African Americans and non-Hispanic whites. In addition, two of the largest studies indicated similar point estimates for the standard effect size () (20
The strengths of this analysis are its inclusion of a variety of study designs, the ability to examine A1C differences by sex and study type, and the use of previously unpublished data (20
). The results of this meta-analysis, however, depend in part on the accuracy and reliability of the A1C measurement across studies. Variability in tests of glycemia has been an issue in the field, and standardization of A1C was not widespread until the last decade (38
). In more recent years, the International Federation of Clinical Chemistry and the National Glycohemoglobin Standardization Program have been working toward a certification of diagnostic equipment as being traceable to the Diabetes Control and Complications Trial reference method (38
). In addition, persistence of hereditary fetal hemoglobin may cause a spurious elevation of A1C (40
). However, type 1 diabetes and insulin treatment seem to be more associated with fetal hemoglobin production (41
Another limitation of the analysis is publication bias; however, we performed numerous searches on this topic and contacted multiple investigators to retrieve the data for A1C means and SDs. The heterogeneity of the studies adds to the limitations of the analysis. Nevertheless, results are likely generalizable to African-American and non-Hispanic white adult patients with type 2 diabetes because the data included a broad range of patient ages, geographic settings, and study types. An additional limitation of this meta-analysis is that despite the comprehensive review of abstracts, the potential for omission exists if an abstract initially reviewed through our search process did not specifically address racial disparities.
The cause of the disparity in glucose control is multifactorial. Previous studies that have examined differences by ethnicity have found significant variation in rates of medical insurance coverage (42
). Other studies of patients with diabetes have found that most have some form of medical insurance and that African Americans and non-Hispanic whites have similar rates of coverage (43
). We were unable to obtain data on insurance status for eight studies included in the meta-analysis; however, two of the largest studies were conducted in managed care settings where all subjects were insured (9
). In these studies, all of the patients had access to health care, but, even then, disparity in A1C levels was seen for minority groups.
Differences in the intensity of treatment may also account for a portion of the variation in A1C control. Determining bias is difficult, and we were unable to examine it in this study. Patient preferences for a type of treatment or comprehension of the disease and its treatment may account for some of the differences. Studies examining acceptance of cardiac procedures and renal transplants have found variation by ethnicity and race; however, differences in preference probably contribute only to a small portion of the variation in care (44
This is the first meta-analysis of racial and ethnic differences in glycemic control among patients with diabetes. Although the studies included used a variety of designs, a consistency in the degree of disparity of glycemic control was found regardless of study type. Multiple separate meta-analyses were conducted across study types and sex with the same outcome. African Americans with diabetes have an ~0.65% higher A1C than non-Hispanic whites; this difference may explain a portion of the excess microvascular complications in this population nationally. We need to understand more fully why this disparity in glycemic control exists and act to eliminate the factors that are modifiable (e.g., improved access to and delivery of care). Further research is needed to elucidate why African Americans with diabetes have poorer glycemic control than non-Hispanic whites and to identify interventions that might prevent or reduce the disparities.