November Q&A: Readers’ questions answered

Oct. 19, 2022

I wonder if you might be able to shed a little light on an issue that we are trying to resolve at our lab. We received a call from a nephrologist about our eGFR equation because we still have a difference in our equation for African-American individuals. I have been researching to find another equation. What I have found so far is a lot of discussion about how this needs to be changed, but I have not actually been able to find anyone with a new equation to provide the best care for all patients. One option would be to just remove the multiplier, but I am not sure that is what is being considered acceptable. Any help that you can provide would be greatly appreciated.

Generating accurate estimates of the glomerular filtration rate (GFR) is not an easy thing to do. Most often, estimates are generated by using serum levels of filtration markers like creatinine and cystatin C, however, there are a number of sources of variability that are unrelated to renal function. Several different GFR estimating equations have been developed in an attempt to deal with these non-renal sources of variability, and although they may improve the estimation of GFR, none of them are perfect.

In some clinical situations, estimates can be quite inaccurate such as when serum concentrations are changing rapidly as with acute kidney injury. Additional characteristics that may alter the accuracy of creatinine-based GFR estimates include degree of patient muscle mass and a high vs. low protein diet. So it is important for clinicians to understand the limitations of estimating GFR across a variety of different patients and clinical scenarios.

The most commonly used equation is the CKD-EPI equation, however, this estimation calculator recently underwent a revision in 2021 in which race was removed. This change was based on an analysis of more than 3,000 participants from four different cohorts in the United States. This 2021 version of the CKD-EPI calculator is officially recommended by the American Society of Nephrology and the National Kidney Foundation, however, clinicians should be aware of situations where the accuracy of this estimate might be negatively impacted and take additional steps to confirm renal function using other methods such as cystatin C or the calculation of creatinine clearance using 24-hour urine collection.

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