New Equation May Improve GFR Estimates

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It is more accurate than the MDRD Study equation at all levels of renal function, researchers report.

PHILADELPHIA— Researchers have developed an equation they believe provides a more accurate way to estimate glomerular filtration rate (GFR).

“Equations to estimate GFR are routinely used to assess kidney function and to detect, evaluate, and manage CKD,” said Andrew Levey, MD, chief of nephrology and professor of medicine at Tufts Medical Center in Boston.

“Our new estimating equation, developed from pooled databases, is more accurate than the widely used Modification of Diet in Renal Disease (MDRD) Study equation.”

Although there is a test to measure the actual GFR, it is time consuming and impractical for routine use, said Dr. Levey, who spoke on the behalf of the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI). The MDRD and similar equations use information on age, race, sex, and the level of creatinine to estimate the GFR.

The MDRD equation has some important limitations, however. It tends to underestimate GFR, especially at GFRs closer to normal. The MDRD was developed using data on about 1,600 patients from a single study, he noted.

Dr. Levey and his colleagues spent nearly three years developing their new equation, which is based on data from more than 8,000 patients in 10 studies. The equation then was validated using data on nearly 4,000 patients from 16 studies.

Compared with actual GFR measurements, the resulting CKD-EPI equation was more accurate than the MDRD equation at all levels of kidney function, although the difference was greatest at higher GFR levels. When the two equations were applied to a nationally representative sample of 16,000 Americans, the estimated prevalence of CKD in the population was 12.2% with the CKD-EPI equation and 13.3% with the MDRD equation.

The National Kidney Foundation estimates that 26 million U.S. adults have CKD and another 20 million are at increased risk. Early detection and treatment are needed to reduce the risk of worsening kidney disease. Estimating equations like the MDRD equation play an important role in CKD diagnosis and treatment.

“The CKD-EPI equation has lower bias, especially at higher estimated GFR,” Dr. Levey explained.  “Although its precision remains limited, the CKD-EPI equation could replace the MDRD equation for routine clinical use.”

However, he said that a single equation may not be accurate in all populations of patients with CKD.  As with the MDRD equation, Dr. Levey cautioned, the CKD-EPI estimates of GFR are not as accurate as actual GFR measurements.

“We think this new equation will avoid some misclassifications and make drug dosing more accurate because we will have the level of kidney function a little more accurate,” Dr. Levey told Renal & Urology News.

In addition, the new equation may help clinicians better monitor changes in progression of kidney disease.  “This equation is now ready for prime time and I think it will have an impact on medical care, research and public health,” Dr. Levey said.

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