Researchers have recently shown that measuring creatinine and cystatin C—two markers for chronic kidney disease (CKD)—more precisely estimates kidney function than measuring either marker alone. Creatinine is a waste product from protein in the diet and the normal breakdown of muscle tissue. Cystatin C is released by cells throughout the body. Normally, the kidneys remove both creatinine and cystatin C from the blood, and they are excreted in the urine. As kidney disease progresses, however, the kidneys do this job less well, leading to increased levels of creatinine and cystatin C in the blood.
Within the kidney, the glomerulus performs the task of filtering waste products and excess salts and fluid from the blood. The “glomerular filtration rate,” or GFR, is a measure of the kidneys’ capacity to filter the blood. However, GFR is rarely measured outside of a research setting. It is most commonly estimated using a mathematical equation that incorporates, among other factors, an appropriate biomarker, usually the level of creatinine in the blood. Physicians and scientists have long known that the method of estimating GFR by measuring creatinine alone is imprecise, because creatinine levels can vary among individuals due to factors that are not related to kidney function, such as differences in muscle mass, malnutrition, or chronic illness. This imprecision can have negative consequences, such as incorrectly classifying patients as having CKD when they may not, leading to unnecessary treatment of healthy individuals. It can also fail to detect the decreased kidney function in patients who do have CKD and who would benefit from treatment. The new study found that using a revised calculation that incorporates both creatinine and cystatin C levels produced more accurate estimates of GFR over a broader range of kidney function and body size than estimates utilizing creatinine alone, and was less likely to be altered by other medical conditions. This research was conducted as part of the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) study, which estimated kidney function in a diverse group of over 5,000 people from 13 studies.
Estimated GFR is routinely reported with the results of the most commonly ordered clinical blood tests. Thus, this CKD-EPI study may have important implications for routine medical care for adults. Furthermore, because there is no effective treatment to restore kidney function once it is lost, prevention and early detection of kidney disease are critically important approaches to prevent kidney failure. These results are an important step towards improving the certainty of CKD diagnosis.
Inker LA, Schmid CH, Tighiouart H, et al. Estimating Glomerular filtration rate from serum creatinine and cystatin C. New Engl J Med 367: 20-29, 2012.