At Risk For Heart Disease?
It has long been known that declining kidney function is a precursor of cardiovascular disease. In fact, the state of your kidney function can be an indication of a potential problem throughout the body. What doctors didn't know was that the standard kidney function test -- serum creatinine -- was not an especially effective measure of kidney function, especially in older people. Now they do. More importantly, researchers at San Francisco VA Medical Center have proof of a far more effective test -- one that measures cystatin C.
WHAT'S WRONG WITH CREATININE TESTING?
Creatinine is a protein waste product of muscle mass, which healthy kidneys clear from the blood. Elevated creatinine in the blood shows that the kidneys aren't functioning properly. But there's a catch -- people who lose muscle mass with age don't produce much creatinine, which means that a blood test will indicate the kidneys are working fine even when they are not. Detecting kidney problems is crucial not only for the possibility of kidney disease but also because declining kidney function is a proven precursor of cardiovascular disease.
WHAT'S RIGHT WITH CYSTATIN C TESTING
Researchers at the San Francisco VA Medical Center recently did a study on a different blood protein waste called cystatin C that the kidneys also clear. Unlike serum creatinine, cystatin C has nothing to do with muscle and so the question was, would measuring the amount of circulating cystatin C be a more sensitive test for kidney function in people over 65 and so of cardiovascular disease risk? The answer was a resounding yes. The study measured creatinine and cystatin C in blood samples gathered in 1992 and 1993 from 4,637 participants aged 65 or older in the Cardiovascular Health Study, a long-term study conducted in four areas of the country. Creatinine measurements showed that only the top 10% of the entire group studied had a significant risk of death from all causes and no independent association of risk of death from cardiovascular reasons. In contrast, the cystatin C test identified low, medium or high risk of death from all causes as well as death from cardiovascular causes. It showed that people with the highest level of cystatin C had a 700% greater risk of death from cardiovascular causes than those with the lowest level. Furthermore, the study established that 60% of the entire group studied had some form of kidney function decline.
INSIGHT FROM THE RESEARCHER
I talked with the study's lead author, Michael Shlipak, MD, MPH, associate professor of medicine, University of California, San Francisco. Now that we know that kidney function often declines with age, it is important to identify kidney dysfunction early and to treat it with appropriate lifestyle changes and, if necessary, medicine. This would no doubt decrease cardiovascular risk and death among the elderly. Yet another value of the cystatin C test, Dr. Shlipak says, is that because it is sensitive to any kidney function decline, it helps determine patient risk for many medical procedures and even surgeries. The reason: Kidney function is directly related to patient risk, and having a realistic evaluation of it will give doctors and patients a more reliable guide in making treatment decisions. The test is FDA-approved for diagnostic purposes now, and although more expensive than the serum creatinine test, Dr. Shlipak says it isn't costly. Unfortunately, only one company makes the test at this time, limiting its availability. Ask your doctor for it anyway. The results will be worth the effort.