When the kidneys are not functioning properly, an individual’s risk for cardiovascular complications is twice as great. Now, there is a lab test – for cystatin C – that may help identify kidney dysfunction earlier and better predict impending cardiovascular problems such as heart attack, stroke, and overall death risk in the elderly. Approved by the FDA for diagnostic use although not yet widely available or commonly used in clinical settings, the cystatin C blood test was recently found to be a more accurate predictor of mortality and cardiovascular risk than creatinine, claims a study in the May 19, 2005 issue of the New England Journal of Medicine (NEJM).

The two kidney function tests were compared in the Cardiovascular Health Study (CHS), a 7-year study of 4,637 participants age 65 and older sponsored by the National Heart, Lung, and Blood Institute and conducted in four communities across the nation. The investigators found that the cystatin C test distinguished those at low, medium, and high cardiovascular risk. Higher cystatin C levels were directly associated with a high risk of death from all causes.

The cystatin C test detects kidney disease at earlier stages, before symptoms appear and creatinine levels rise. For the elderly and those with diabetes, hypertension, or cardiovascular disease, the new test may thus help detect disease earlier and facilitate prevention efforts. Another advantage is that, unlike creatinine, blood levels of cystatin C are less influenced by age, gender, race, or lean muscle mass, which makes this a better indicator of kidney function.

With greater study and use, the test’s exact clinical role will be better determined. In an article in Medical News Today, the CHS study leader noted that the test may be most useful in assessing preoperative risk and the risk of side effects from medication and from dyes used in radiology procedures. However, further research is needed, especially to verify the efficacy of cystatin C as a diagnostic test for kidney disease. It will also be important to further examine the findings from this particular study. As Drs. Stevens and Levey point out in their editorial in the same issue of NEJM, other factors besides the glomerular filtration rate (GFR), which is based on levels of serum and urine creatinine, may have confounded the findings. Although studies have shown that cystatin C can be a better index of GFR than serum creatinine alone, it may not be any improvement over creatinine-based GFR estimates that take into account patient age, gender, and weight.

Sources
New kidney function test better than standard at predicting death and cardiovascular outcomes (press release). National Heart, Lung, and Blood Institute. 18 May 2005. On the Internet at: http://www.nhlbi.nih.gov/new/press/05-05-18.htm.

Shlipak MG, Sarnak MJ, Katz R et al. Cystatin C and the risk of death and cardiovascular events among elderly persons (abstract). NEJM. 19 May 2005. 352:2049-2060. On the Internet at: http://content.nejm.org/cgi/content/abstract/352/20/2049.

New kidney test better elder mortality predictor, tests for cystatin. Medical News Today. 19 May 2005. On the Internet at: http://www.medicalnewstoday.com/medicalnews.php?newsid=24722.

Stevens LA and Levey AS. Chronic Kidney Disease in the Elderly — How to Assess Risk. NEJM Volume 352 (20):2122-2124, May 19, 2005. On the Internet at: http://content.nejm.org/cgi/content/short/352/20/2122