One glomerular filtration rate equation may offer better tradeoffs than the other, review indicates

While neither of the two main equations for calculating glomerular filtration rates (GFRs) is universally better, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation may be more useful for patients in North America, Europe and Australia, a study concluded.

Researchers compared the CKD-EPI equation to the Modification of Diet in Renal Disease (MDRD) equation to review their performance and to inform the selection of a single equation by laboratories and clinicians. A systemic review of the literature from 1999 to October 2011 found cross-sectional studies in adults that compared the equations' performances.

Results appeared online Feb. 6 at Annals of Internal Medicine.

In adult populations in North America, Europe or Australia, researchers reviewed 12 studies comprising 12,898 patients. The CKD-EPI equation was more accurate than the MDRD equation in 10 studies and less accurate in two studies. Bias ranged from 14.6 to −22 mL/min per 1.73 m². The CKD-EPI equation was less biased than the MDRD equation in seven studies and more biased in five studies. In six of the 10 studies that reported a measure of precision, the CKD-EPI equation was more precise than the MDRD equation. Precision of the MDRD was better or the same in the other four studies. In five studies, performance measures were consistently better for the CKD-EPI than for the MDRD. In two studies, both of which were conducted in kidney transplant recipients, performance measures were better for the MDRD.

In adult populations in Asia and Africa, eight studies examined GFR estimations. In six studies, the MDRD or CKD-EPI equation was modified by adding or removing a coefficient to improve performance or a new equation was developed using the same variables. Comparing the performance of equations across studies was difficult because locally derived equations were generally not tested in other studies or populations. In these studies, the unmodified MDRD and CKD-EPI equations were less accurate than they were in the studies in North America, Europe and Australia. Coefficients developed in one ethnic or racial population did not improve equation accuracy in a study of another ethnic or racial population.

“The observed and expected differences in performance by range of GFR suggest that we cannot optimize the performance of any equation for all clinical populations across a wide range of GFRs,” the authors wrote. “Because the goal is to select a single estimating equation for routine use by clinical laboratories, the tradeoff of optimizing performance at either higher or lower GFR ranges must be accepted. Because the difference in bias (on the raw scale) between the equations is greater at higher GFRs, using the CKD-EPI equation would lead to smaller average bias in clinical populations with a wide range of GFRs.”

Applying the CKD-EPI rather than the MDRD would potentially allow more efficient use of resources in caring for patients with lower estimated GFR, the authors noted. And it would allow reporting of estimated GFR as a numerical value throughout the full range, rather than limit it to lower values (for example, <60 mL/min per 1.73 m², as currently recommended for the MDRD in the United States). However, using the CKD-EPI equation would slightly increase overestimation at lower GFRs. Nephrologists and others caring for patients with low GFR would need to be aware of this limitation, the authors concluded.