Estimating Kidney Function in Healthy People

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The summary below is from the full report titled "Using Serum Creatinine To Estimate Glomerular Filtration Rate: Accuracy in Good Health and in Chronic Kidney Disease." It is in the 21 December 2004 issue of Annals of Internal Medicine (volume 141, pages 929-937). The authors are A. Rule, T.S. Larson, E.J. Bergstralh, J.M. Slezak, S.J. Jacobsen, and F.G. Cosio.

What is the problem and what is known about it so far?

The most accurate way to assess kidney function is to measure a person's glomerular filtration rate (GFR). The GFR shows how well the kidneys filter waste from the blood. Measuring GFR requires injecting a substance into the bloodstream and seeing how well it is cleared from the body with a 24-hour urine collection. Another option is to estimate GFR by using formulas that include a patient's age, sex, and creatinine value (a simple blood test that measures waste products). One of these formulas is called the Modification of Diet in Renal Disease (MDRD) equation. The ability of this equation to estimate GFR has been tested in patients with kidney disorders, but we do not know how well it performs in relatively healthy people without known kidney disease.

Why did the researchers do this particular study?

The researchers wanted to see if the MDRD equation estimates GFR equally well in patients with and without known kidney disease. They also wanted to find a new equation that would estimate GFR better than the MDRD equation.

Who was studied?

320 patients age 17 to 87 years with chronic kidney disease and 580 healthy kidney donor candidates age 18 to 72 years.

How was the study done?

The researchers reviewed medical records of adults who were seen at the Mayo Clinic in Rochester, Minnesota, from 1996 to 2003 either for chronic kidney disease or as potential kidney donors. They found people who had had blood tests for creatinine and GFR measurements (iothalamate clearance tests). They calculated the MDRD equation estimate of GFR based on each person's age, sex, and creatinine value. They then compared the MDRD equation estimate with the actual GFR measurement in both groups of patients. Finally, they formulated a higher-order equation (a quadratic equation) that combined the age, sex, and creatinine values in a different manner and tested its ability to estimate GFR.

What did the researchers find?

The MDRD equation underestimated kidney function by small amounts (6%) in the patients with kidney disease and by large amounts (29%) in the healthy adults. Among the healthy adults, the higher-order equation better estimated renal function than did the MDRD equation.

What were the limitations of the study?

The study included only 7 African Americans and few people older than 65 years of age. The proposed new equation might not work well in these patients. It should not be used routinely until it has been tested in larger and more diverse populations.

What are the implications of the study?

The MDRD equation underestimates kidney function. Using it may mistakenly label some healthy individuals as having kidney disease.