In this population of socioeconomically similar black and white SCCS participants, the age-adjusted incidence of ESRD was more than 3.5 times higher among black than whites. Even after controlling for well-defined socioeconomic factors and extensive demographic, medical and lifestyle information collected at study entry, including comorbidities such as diabetes and hypertension, blacks continued to have a 2.4-fold greater risk of ESRD relative to whites. It is noteworthy that the strengths of the associations of the various factors with risk of ESRD tended to be the same for blacks and whites. Thus, although residual confounding cannot be ruled out entirely, the examined socioeconomic and other risk factors for ESRD do not explain the observed substantially higher incidence of ESRD observed among blacks.
Our data are consistent with national statistics 
and observations of higher rates of ESRD observed among blacks in other study populations 
. A small number of prospective studies have had sufficient sample size and length of follow up to directly compare risk factors for incident ESRD among blacks and whites. In the Atherosclerosis Risk in Communities (ARIC) study 
of 3,954 blacks and 11,370 whites, older age, smoking, male sex, diabetes and hypertension were positively associated with incident ESRD, and African American race remained a strong predictor even after controlling for these factors, with a HR of 2.5. These results are very similar to what we found in the SCCS, and to results of the Multiple Risk Factor Intervention Trial (MRFIT) and a study among Medicare beneficiaries aged 66 years or older with hypertension or diabetes. 
. Similarly, in. In a large population of veterans 
, including 311,790 blacks and 1,704,101 whites, the incidence of ESRD was consistently higher among blacks than whites at all levels of baseline kidney function, after adjustment for socioeconomic variables and comorbidities. Although that study included a larger number of ESRD cases (N
4,379 cases among blacks, 10,769 among whites) than our study population, it did not report associations of ESRD with characteristics of the study participants other than estimated glomerular filtration rate (eGFR). Most recently, in the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, a total of 133 incident cases of ESRD were observed over a median of 3.6 years of follow-up among 27,911 participants 
. ESRD rates among blacks and whites were comparable to those observed in the current study, but the fourfold greater risk of developing ESRD among blacks compared with whites decreased to 1.4 after adjustment for hypertension, diabetes, income, and education, as well as eGFR and albumin-to-creatinine ratio.
A striking pattern of racial differences exists also for CKD, and provides evidence that the observed disparity in ESRD incidence between blacks and whites may be due, at least in part, to racial differences in the rate of progression of CKD to ESRD. In the San Francisco Community Health Network study population, most similar to the SCCS population in terms of socioeconomic distribution, blacks with CKD stages 3 to 5 had a four-fold higher risk of progression to ESRD than whites after adjustment for socioeconomic status 
. The prevalence of early stage CKD is higher among whites compared with blacks 
, with detailed analyses based on eGFR measurements showing that the white excess for mild CKD gives way to an excess for moderate to severe CKD in blacks 
. It has been hypothesized that this “cross-over” pattern may be due to lower mortality at higher GFR levels among blacks. Empirical evidence, however, demonstrates that death rates are higher among blacks than whites at all levels of CKD prior to ESRD 
, and rates of ESRD incidence are nearly five times higher than rates of cardiovascular death among blacks with hypertension 
, and thus does not lend support to this explanation. More intense surveillance among whites than blacks could contribute to elevated prevalence among whites of early stage CKD, for whom symptoms are less marked, or, on the contrary, whites may become symptomatic at higher GFR levels and thus have CKD diagnosed earlier. Shorter sojourn times in early stage disease by blacks than whites may also be involved, but explanations for speculated faster transitions to moderate and advanced CKD and potentially to ESRD for blacks remain clear.
We did not routinely measure baseline serum creatinine for estimation of GFR among SCCS participants, but blood samples were collected from over half of those who enrolled in CHCs. As part of ongoing nested case-control studies for breast, lung, colorectal and prostate cancer, independent of the current ESRD analysis, prediagnostic blood from 1,593 cases and age- and race-matched controls was assayed for several biomarkers, including serum creatinine. Wwe found that the mean creatinine levels were about 3% higher among black (1.10 mg/dl) than white men (1.07 mg/dl) and 14% higher among black (0.90 mg/dl) than white women (0.79 mg/dl), consistent with reports in other populations of blacks and whites 
. Using the CKD-EPI equation to estimate GFR 
, the percentages in the SCCS sample of participants with eGFR of >90 (“normal”), 90–61 and ≤60ml/min/1.73 m2
were 56.9%, 34.5% and 8.6% among black men, 39.5%, 51.3% and 9.2% among white men, 58.7%, 32.5% and 8.8% among black women and 47.7%, 39.5% and 12.8% among white women. These figures suggest that the overall prevalence of CKD at entry into the SCCS may be lower among blacks than whites. For severe CKD, defined by eGFR <30 ml/min/1.73 m2
, a modest excess among blacks became apparent, with prevalences of 1.9%, 1.8%, 1.7% and 0.4% among black men, black women, white men, and white women, respectively, although based on very small numbers of study subjects. However, the observed excess in the incidence of ESRD among blacks in our study population was much greater than the excess in estimated severe CKD. Twenty-four of the incident ESRD cases in this study were among those who had existing baseline serum creatinine measurements; for these eGFR at study entry was below 90 ml/min/1.73 m2
for 21 (88%) and below 60 ml/min/1.73 m2
for 16 (67%). Further assessment of CKD is beyond the scope of this analysis, but additional research is needed using serum and urinary biomarkers collected at baseline, in conjunction with repeat blood collections to measure progression rates and times by race among SCCS participants with CKD.
While awareness and treatment of hypertension has improved among blacks over time, a substantially higher prevalence of hypertension among blacks compared with whites has been reported to persist, as well as the difference between blacks and whites in the proportion of patients with hypertension who are receiving treatment 
. In the overall SCCS population, the prevalence of self-reported hypertension at baseline was 59% among blacks, significantly higher than the 50% among whites, but approximately 80% of both blacks and whites with hypertension reported use of anti-hypertensive medications. It has been suggested that a lower blood pressure level may be necessary to slow the decline of renal function among blacks with CKD and hypertension compared with whites 
, but racial differences in the susceptibility to renal damage from elevated blood pressure have been reported to persist even after adjustment for differences between blacks and whites in hypertension and hypertension-control 
. In our study, the HR estimate associated with hypertension was somewhat (but not significantly) higher among blacks than whites (HR
3.0 versus 2.1), which lends some support to the view that the higher incidence of ESRD in blacks may be attributable in part to a greater sensitivity to the effects of elevated blood pressure among blacks. With respect to diabetes, the onset of albuminuria, hypothesized to be an etiologic factor rather than simply a biomarker 
, appears to present earlier in the course of diabetes in blacks compared with whites, which may suggest more aggressive disease progression 
, but this is unlikely to explain the similar HR estimate observed between diabetes and ESRD among whites and blacks in our study population.
The SCCS is a unique cohort in which to study health effects in blacks compared with whites both because of the large number of blacks and the comparability of socioeconomic status between the racial groups. In addition, the collection of extensive baseline information for the entire SCCS cohort and the unbiased and virtually complete follow up for ascertainment of ESRD 
are major strengths of our study. Over 85% of our study population was drawn from CHCs, which are expected to provide race-neutral care for diabetes, hypertension and other chronic illnesses. Limitations include the lack of data on baseline kidney function or proteinuria for the ESRD cases, the self-reported nature of the questionnaire data, and the lack of time-dependent covariates. However, we conducted internally valid comparisons of HRs for ESRD between blacks and whites, and our findings support the existing literature on this subject and warrant further etiologic research in future prospective studies of ESRD in blacks.
While race appears to be an independent predictor of ESRD, and possibly of more rapid loss of kidney function among those with moderate and advanced CKD 
, longitudinal studies with repeated measures of kidney function are critically needed to evaluate whether stage-specific kidney disease may progress more rapidly in blacks than in whites and to identify genetic, environmental or behavioral factors that may explain these differences in progression rate and incidence of ESRD among blacks in the United States. A recent study 
showed that, among those of recent African ancestry, focal segmental glomerulosclerosis and hypertension-attributed ESRD are strongly associated with two coding sequence variants in the APOL1 gene on chromosome 22. The APOL1 genotype has also been shown to associate with microalbuminuria among nondiabetics 
, and with younger age at initiation of hemodialysis among nondiabetic blacks with ESRD 
. The APOL1 risk alleles for renal disease occur in more than 30% of those with recent African ancestry, but to date have not been observed in European Americans 
. Thus, these emerging data support the existence of a relatively common, high-risk genotype that confers susceptibility to nondiabetic kidney disease in African Americans 
Our study has demonstrated that, after taking into account traditional risk factors strongly associated with ESRD among both blacks and whites, blacks continue to have a substantially increased risk for ESRD compared with whites. Further research characterizing CKD progression and ESRD occurrence by race may help greatly in clarifying the natural history and etiologic events leading to ESRD in these two populations.