WEDNESDAY, May 18, 2005 (HealthDay News) -- A new test of kidney function may be a better indicator of impending cardiovascular problems such as heart attack, stroke and overall death risk in elderly patients, a new study suggests.
The new screen measures blood levels of cystatin C, a tissue-produced protein that is steadily removed from the bloodstream by the kidneys. High levels of circulating cystatin C would seem to indicate that kidneys aren't doing the job they should.
"We think it is giving us a better reflection of kidney function, and a better test of kidney function could have a tremendous potential for clinical applications," said lead researcher Dr. Michael G. Shlipak, an internist at the San Francisco Veterans Affairs Medical Center and a professor of medicine and epidemiology at the University of California, San Francisco.
His team reports the findings in the May 19 issue of the New England Journal of Medicine.
The study compared the predictive value of cystatin C blood levels vs. creatinine levels. Creatinine, a protein made by muscle cells that is also removed by the kidneys, is what doctors now measure to assess kidney function.
The researchers tracked outcomes in more than 4,600 older patients followed for nearly a decade. They found a direct relationship between higher levels of cystatin C and future cardiovascular problems and death.
In contrast, blood levels of creatinine did not provide as clear a link, the researchers reported.
A commercial blood test for cystatin C is available, but is not used in medical practice, Shlipak said. He believes the study is a good first step toward the more widespread use of the test by physicians.
The screen could be especially useful for older people, Shlipak said, since levels of circulating creatinine are affected by the steady loss of muscle that occurs with age. Still, a lot must be done before widespread use of the cystatin test is possible, he cautioned.
"First, we need to confirm that our findings are not unique to this population," he said. "We need another cohort study to confirm the finding."
Such a study is nearing completion. Although Shlipak would not disclose the results in detail, he described them as "comforting."
Next, he said, "we must test whether we can improve clinical management with this test. That would take a while, because it would need a large study. We would need some big money to do it."
If all those requirements are met, Shlipak sees a gradual introduction of the cystatin C test into medical practice, perhaps in coordination with creatinine testing, especially for people with known cardiovascular risks such as diabetes and high blood pressure.
"I would start with elderly people," he said. "I would probably create a creatinine-first strategy. If the creatinine level is high, you know there may be a problem. But even if it looks normal, I don't think you can be confident enough that you have normal kidney function, so I would then do a cystatin C test."
Dr. Lesley Stevens, an associate professor of medicine at Tufts University School of Medicine, wrote an accompanying editorial. She believes the study failed to answer one basic question.
"They make a statement that the cystatin C test is a better predictor of [patient] outcome," she said. However, "the question is whether cystatin C is a better indicator of kidney function than creatinine, and this study does not address that question. You want to know the level of kidney function to guide overall treatment, not just treatment for cardiovascular disease."
More work must be done to show that the cystatin C screen beats current creatinine testing in measuring kidney function, Stevens said. Still, she said, "the most important message here is that regardless of whatever test you use, chronic kidney disease is an important measure of risk of cardiovascular disease."
Anything you want to know about kidney disease is available from the National Institute of Diabetes and Digestive and Kidney Diseases.