Study of Black-White Differences in Anemia Prevalence in the United States: The Reasons for Geographic and Racial Differences in Stroke Study.

Introduction: Anemia by the WHO criteria is more common in American blacks than whites. There are few data examining differential associations of demographic, socio-economic, and co-morbid conditions with anemia by race.

Methods: The Reasons for Geographic and Racial Differences in Stroke (REGARDS) study is a national longitudinal cohort currently enrolling 30,000 blacks and whites aged ≥ 45 years to assess stroke risk. Half the cohort will be black and half will live in the southeast. A phone interview for health history was followed by an in-home visit for phlebotomy and physical exam. After enrolling 8400 subjects, a complete blood count was added to the baseline exam. As of March 30^th 2006 this was available in 12,060 participants. Anemia was defined by the WHO criteria and categorized into 3 non-exclusive groups: (1) low glomerular filtration rate (GFR) (<60ml/min/1.73m²), (2) inflammation (C-reactive protein ≥ 10mg/L or leukocyte count ≥ 15×10⁹/L, and (3) microcytosis (mean red cell volume < 80fL).

Results: The prevalence of anemia was 13.3% (1600 of 12,060); 21.5% for blacks (1047 of 4860) and 7.7% for whites (553 of 7195). The age- and sex-adjusted OR of anemia for blacks vs. whites was 3.71 (95% CI 3.30, 4.18). After additional adjustment for socio-economic variables (high school education, annual income), and co-morbid conditions (vascular disease, diabetes, hypertension, ever smoker, body mass index), blacks had a 2.79-fold (95% CI 2.44, 3.19) greater prevalence of anemia. Correlates of anemia differed by race; compared to whites, blacks with anemia were younger (66.2 vs. 69.9 years old, p <0.01), more likely to be female (70% vs. 58%, p <0.01), and less likely to live in the southeast (52% vs. 66%, p <0.01) with lower socio-economic indicators for blacks vs. whites (74% vs. 89% for high school education; 37% vs. 21% for annual income < $25,000; both p <0.01). Blacks with anemia, despite a higher prevalence of diabetes (42% vs. 32%, p <0.01) and hypertension (78% vs. 65%, p <0.01) than whites, had a lower prevalence of vascular disease (27% vs. 39%, p <0.01). Anemia type differed by race; fewer blacks than whites had anemia with a low GFR (45% vs. 67%, p <0.01)., while more blacks than whites had anemia with inflammation (18% vs. 14%, p = 0.04) or microcytosis (22% vs. 10%, p <0.01). Addition of anemia type (low GFR, inflammation, and microcytosis) to the multivariable model did not alter the association of black race with anemia (HR 2.71, 95% CI 2.35, 3.13).

Conclusions: Anemia is more common in blacks than whites and correlates of anemia differ greatly by race. While risk factors for low GFR were more common in blacks (diabetes, hypertension), anemia with low GFR was more prevalent in whites. Adjustment for demographics, socio-economic variables, co-morbid conditions, and anemia type did not eliminate the association of black race with anemia. Whether this represents intrinsic differences in hemoglobin concentration between blacks and whites or undetermined or unmeasured co-morbid conditions requires further study.