Diagnostics & Imaging Week Washington Editor

A report from the National Institutes of Health (NIH; Bethesda, Maryland) says a new blood test for kidney function is better at predicting death and cardiovascular risk in the elderly.

The test, called cystatin-C, is more sensitive, according to the NIH's National Heart, Lung and Blood Institute (NHLBI), which funded the study. The study shows the test is able to distinguish patients at low, medium, and high cardiovascular risk, allowing for much earlier detection, NHLBI said.

"What's really encouraging right now is that this adds to the body of knowledge showing a relationship between kidney disease and cardiovascular events," Jean Olson, MD, project officer for NHLBI's Cardiovascular Health Study, told Diagnostics & Imaging Week.

Cystatin-C has been FDA-approved for diagnostic use, but the test is not yet widely available or commonly used in clinical settings.

Researchers compared the two measures of kidney function cystatin-C and creatinine, the standard test as predictors of death from all causes, death from cardiovascular causes, and incidence of heart attack and stroke among the 4,637 elderly participants in the observational study.

Roughly 20% 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% higher risk of heart attack and stroke compared with those who had the lowest levels of cystatin-C.

Testing the same participants with creatinine detected a smaller high-risk group about 10% of the participants and all others appeared to be at average risk.

When cystatin-C was used, investigators found that 60% of test subjects had abnormal kidney function putting them at medium or high risk for cardiovascular complications.

"Our results show that a normal creatinine is not nearly as reassuring as we used to believe," said Michael Shlipak, MD, lead author of the study and professor at the University of California, San Francisco.

With people at a high risk for kidney disease such as the elderly or those with diabetes, hypertension or cardiovascular disease a normal creatinine level could be "misleading us into thinking that the patient is safe from the cardiovascular effects of kidney disease," Shlipak said.

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, the report said.

"Going forward, the next step would be to run similar tests in different subsets of the populations, including young adults," Olson said. "In the future we would want to see if cystatin-C is indeed valuable as a better predictor and that the strong relationship holds."

She said more studies would have to be done before the test was widely adopted clinically, which Olson said could take years to complete.

"Eventually someone will put together clinical guidelines making recommendations," she added.

In addition to more research being needed, the test also will find greater clinical use once it is considered "reimbursable," Olson said.

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, providing a clue to physicians that a person may have kidney disease.

Because creatinine is a byproduct of muscle cells, its levels in the blood can be affected by factors other than kidney disease, such as 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.

Additional research will be needed to determine the exact clinical role for the test, Shlipak said. Evaluating the mechanisms that underlie this strong association between the kidney and cardiovascular disease would be critical for targeting prevention efforts, he added.

Detailed results of the study are published in the May 19 issue of the New England Journal of Medicine.

Roughly 20 million Americans have significantly reduced kidney function and even a small loss of kidney function can double a person's risk of developing cardiovascular disease, according to the NIH.