New Kidney Test Better Than Creatinine

A new blood test for kidney function is a better predictor of death and cardiovascular risk among the elderly than the standard creatinine test, according to a study published in the May 19, 2005, issue of The New England Journal of Medicine.

The Cystatin-C test is more sensitive than creatinine, distinguishing patients at low, medium, and high cardiovascular risk. Since Cystatin-C is produced by blood cells, its levels in the blood are not affected by age, gender, race, or lean muscle mass. In contrast, blood levels of creatinine, a by-product of muscle cells, can be affected by all those factors.

The standard evaluation of kidney function is an estimate of the kidney's rate of filtration, called the glomerular filtration rate (GFR). This estimate is based on the measurement of creatinine in the blood, and a further calculation based on the patient's age, gender, and race.

Investigators for the U.S. National Heart, Lung, and Blood Institute (NHLBI, Bethesda, MD, USA) compared the two tests as predictors of death from all causes, death from cardiovascular causes, and incidence of heart attack and stroke among more than 4,000 elderly participants. The 20% of participants with the highest levels of cystatin-C had twice the risk of death from all causes as well as death from cardiovascular disease, and they had a 50% higher risk of heart attack and stroke compared to those with the lowest levels. In contrast, when the same participants were tested with creatinine, it detected a smaller risk group of about 10% of the participants, while all others appeared to be at average risk.

Other studies have shown that cystatin-C may detect moderate kidney disease at earlier stages, before creatinine levels would rise, enabling doctors to identify a much larger group of people at risk for death and cardiovascular complications.

"This study affirms the important link between kidney function and cardiovascular health and survival in the elderly,” observed Elizabeth G. Nabel, M.D., director of the NHLBI. "If these findings are confirmed in other studies, cystatin-C could be a useful prognostic tool for evaluating older people at risk for not only kidney disease but cardiovascular disease as well.”