April 18 (HealthDay News) -- Differences in muscle mass related to younger age or body composition may not explain why black patients with advanced kidney disease have higher levels of creatinine than white patients, says a U.S. study that challenges a widely held belief.

Creatinine is a standard indicator of kidney function. A higher level of creatinine is generally a sign of lower kidney function.

In this study, researchers at the University of California, San Francisco School of Medicine examined serum creatinine concentrations and body composition in more than 3,000 kidney dialysis patients. As expected, blacks had higher creatinine levels than non-black patients.

"A widely assumed explanation for this racial difference is that black patients tend to develop end-stage kidney disease earlier than whites. So blacks on dialysis tend to be younger than whites and persons of other races and ethnicities and so may have more muscle mass, and creatinine is a natural breakdown product of muscle," study leader Dr. Joy Hsu said in a prepared statement.

To test this theory, Hsu and colleagues used something called bioelectrical impedance analysis to estimate the patient's muscle mass.

"We hypothesized that adjusting for muscle mass and related factors would eliminate or reduce the racial differences in serum creatinine levels," Hsu said.

However, even after making that adjustment, black patients had significantly higher creatinine levels.

"The higher creatinine levels in black patients compared to non-black patients could not be entirely explained by differences in age, sex, body size, or muscle mass," Hsu said.

The study was published online in the Clinical Journal of the American Society of Nephrology and is expected to be published in the July print edition of the journal.

More research is needed to uncover why black patients with kidney disease have higher creatinine levels than other patients. The "answer or answers to this question may help explain why blacks are affected by progressive chronic kidney disease more so than whites," Hsu said.

SOURCE: American Society of Nephrology, news release, April 16, 2008