Cystatin-C, a new blood test for kidney function, is a better predictor of death and cardiovascular risk among the elderly than the standard measure of kidney function, according to a National Heart, Lung, and Blood Institute (NHLBI)-funded study published in the current issue of the New England Journal of Medicine. This more sensitive test distinguishes those at low, medium and high cardiovascular risk, which may enable earlier detection.
Investigators for NHLBI's Cardiovascular Health Study compared the two measures of kidney function, cystatin-C and the standard test creatinine, as predictors of death from all causes, death from cardiovascular causes, and incidence of heart attack and stroke among 4,637 elderly participants in the study.
The 20 percent of the 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 a 50 percent higher risk of heart attack and stroke compared with those who had the lowest levels of cystatin-C. In contrast, testing the same participants with creatinine detected a smaller high-risk group--about 10 percent of the participants--and all others appeared to be at average risk.
With cystatin-C, investigators found that 60 percent had abnormal kidney function putting them at medium or high risk for cardiovascular complications.
It is estimated that 20 million Americans have significantly reduced kidney function, and that even a small loss of kidney function can double a person's risk of developing cardiovascular disease.
"This study affirms the important link between kidney function and cardiovascular health and survival in the elderly. 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," said Elizabeth G. Nabel, M.D., NHLBI director.
The standard evaluation of kidney function is an estimate of the kidney's rate of filtration--called the glomerular filtration rate (GFR)-- based on measurement of creatinine in the blood and a further calculation based on a patient's age, gender and race. Measurement of cystatin-C in the blood also appears to reflect the GFR, but does not require an additional calculation. Both creatinine and cystatin-C are proteins found in the blood and filtered through the kidneys. When the kidneys are not working well, these proteins accumulate in the blood, which provides a signal to the doctor that a person may have kidney disease. Because creatinine is a by-product of muscle cells. Its levels in the blood can be affected by factors other than kidney disease, like age, gender, race, and lean muscle mass. Cystatin-C is produced by blood cells, and its levels in the blood are not impacted by age, gender, race, or lean muscle mass.