It’s long been known that black Americans are four to five times as likely as white Americans to suffer from kidney disease that is severe enough to require dialysis or transplantation. Now a new study conducted by researchers at the University of California, San Francisco (UCSF) and San Francisco VA Medical Center (SFVAMC) shows that rates of early kidney disease do not differ between the two groups, but that blacks with early disease are five times as likely as whites to progress to the severe stage that requires dialysis or transplantation. The findings suggest that black Americans with early kidney disease require much more aggressive treatment than has been commonly practiced.
“Black Americans make up a third of the dialysis population, but only about a tenth of the overall population. But nobody has ever looked at whether blacks have more early kidney disease than whites,” says Chi-yuan Hsu, MD, lead author of the study and an assistant professor of medicine at UCSF. “What we found is that the two groups have the same amount of kidney disease, but if someone has early disease and they are white, their chances of going to dialysis are one in 100 per year. But if they are black, their chance is five times higher.”
The study appears in November’s Journal of the American Society of Nephrology. Senior investigator in the study is Michael Shlipak, MD, MPH, assistant professor of medicine, epidemiology and biostatistics at SFVAMC and UCSF.
In the United States, the two major risk factors for kidney disease are uncontrolled diabetes and hypertension (high blood pressure). Early kidney disease, also called chronic renal insufficiency, is a gradually progressing disease that if left untreated can result in end-stage renal disease, the point at which kidneys fail. Without a kidney transplant or dialysis—a blood-filtering process that must be performed several times a week—a person suffering from end-stage kidney disease will die. Although statistics for end-stage kidney disease and its treatments are tracked through a comprehensive national registry (the U.S. Renal Data System), early-stage kidney disease can go undetected for years, making its onset and progression difficult to detect.
In order to determine the prevalence of the earlier stages of disease, Hsu and his team turned to a national health survey conducted periodically by the Centers for Disease Control called NHANES, or the National Health and Nutrition Examination Survey. The survey uses interviews and a battery of tests to assess the health of a representative sample of people across the country. Survey results are then extrapolated to the U.S. population as a whole.
Using records from blood tests drawn during the 1988-1994 survey, Hsu and his team used a formula to estimate the glomerular infiltration rate—a measure of kidney function—of each of the survey’s black and white participants aged 20 to 74, a total of 13,351 people. They found that about 2 percent of blacks and 2.5 percent of whites had early kidney disease, a difference that was not statistically significant. To estimate how many people progressed to end-stage kidney disease five years later, they examined U.S. Renal Data System records for the number of new cases of end-stage kidney disease that arose in 1996 among black and white adults 25 to 79 years old. They found that for each 100 white people identified as having early kidney disease in 1991 (the mid-point of the NHANES survey), one went on to develop end-stage disease in 1996. But for each 100 black people with early kidney disease in 1991, five developed end-stage disease in 1996.
“We were very surprised to find that blacks didn’t have more early kidney disease than whites,” Hsu says. “We thought we would find that they had more kidney disease at all the stages.”
Black Americans are more than twice as likely as white Americans to have hypertension, and one-and-a-half to two times as likely to have diabetes. Hsu found that black participants with early kidney disease in the NHANES survey had higher blood pressure than whites, with average systolic and diastolic pressures of 147 and 82, versus 136 and 77. Yet he also found that diabetic blacks and white survey participants with early kidney disease had similar blood sugar levels (an indication of how well diabetes is being controlled). Differences in blood pressure are unlikely to completely explain the five times higher rate of progression to end-stage disease, Hsu says. “We don’t know the causes for this disparity,” he says. “It could be quality of care and access to health care.” A genetic component can’t be ruled out, either, he says.
Based on this study’s findings, Hsu and colleagues suggest that the National Kidney Foundation modify its Chronic Kidney Disease classification system to take into account the risk of progression of the disease. Right now a black person and a white person with chronic renal insufficiency would both be classified as having stage 3 or 4 chronic kidney disease. “But the implications of suffering stage 3 or 4 disease are very different for blacks and whites,” Hsu says. “Our study suggests that a black person even with mild kidney disease should be treated much more aggressively. They should certainly attain at least as good blood pressure control, and they may need much more close monitoring and follow-up. The disparities we found in blood pressure control and progression rate from early to end-stage kidney disease must be addressed.”
More than 400,000 Americans with end-stage renal disease are being treated by dialysis. With the country’s population growing older and rates of diabetes soaring in large part due to increasing incidence of obesity, this number has more than doubled in the past decade, and is projected to grow to 650,000 by 2010, costing the Medicare system alone $28 billion.
Other investigators in the study were Feng Lin, MS, and Eric Vittinghoff, PhD, both from the department of Epidemiology and Biostatistics at UCSF. The study was funded by the National Institutes of Health and the Department of Veterans Affairs.