GFR for Everybody?

Use of prediction equations for inpatients is problematic.

Most labs report an estimated GFR (glomerular filtration rate) using a formula such as the Modification of Diet in Renal Disease (MDRD) equation. These are generally accurate for average-sized outpatients 18-70 years, and provide an easy way to help physicians gauge kidney function.

Use of prediction equations for inpatients is problematic because of the non-steady state of creatinine in these patients. As CAP consultant Gregory Miller, PhD, told CAP Today, “…on inpatients the estimating equation is less accurate, and with acute renal failure it’s not really appropriate.” Additionally, equations are less accurate in pregnant patients, bodybuilders and trained athletes.

To make matters worse, many physicians—and most pharmacists—believe that the Cockcroft-Gault equation is more accurate than the MDRD and others because it includes weight. It does not, however, correct for body surface area, sex or race.

This can create something of a problem for labs, especially if physicians assume the MDRD equation is appropriate for dosing medication in all cases or can be applied to all patients. Your information system may or may not be able to suppress the estimated GFR. If it can, it might be a good idea to do so for patients less than 18 or greater than 70 years. Some systems can suppress based on patient type, another possible solution.

But (of course) any calculated answer is only a calculator or website away. I had a recent conversation with an imaging tech who complained that we reported an “inaccurate” GFR for purposes of giving contrast medium.

Lastly, I’ve spoken to physicians who think the MDRD equation is inferior to the Cockcroft-Gault, who think both are limited or who aren’t interested one way or another. Some think a disclaimer on the report that the MDRD should not be used for medication dosing is appropriate. Others I’ve heard from don’t want it reported for elderly patients, and I see the point: it’s unlikely for a patient over 80 to have a normal GFR.

The calculation isn’t at fault (I’ve read there are 46 different equations); they are likely to be of limited use, especially on inpatients or in atypical settings. A laboratory reporting these calculated estimates is also not at fault, since that is a recommended best practice. Adding disclaimers sounds good, but what disclaimers? A result is either usable or not as deemed by the ordering physician, and adding a treatment recommendation is outside the scope of laboratory medicine, even dangerous. And while it can be damaging to a laboratory’s reputation to have pharmacists, physicians and other members of the team claiming lab reports are inaccurate, but that isn’t a reason to change what we do.

I’m not sure what the best answer is other than “educate physicians.” This issue is no different than other calculations such as MCH, MCHC, anion gap, and others. Estimates of are often crude, and there is no way around that. But they can also be useful, depending on the setting. Just the same, I feel it’s probably appropriate to suppress prediction equations for inpatients, the young and old—and probably pregnant patients to be safe—but it all depends on what the docs want.

NEXT: Communicate This and That

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