CKD-EPI 2021 vs MDRD: Formula Changes and Comparing Lab Reports

An eGFR change between two reports can reflect a change in the estimating equation, as well as a change in kidney function. CKD-EPI 2021 is not simply the old equation with its race multiplier deleted: it was refitted without a race variable. Our calculator uses this 2021 creatinine equation, while a laboratory elsewhere may follow a different local recommendation.

What differs between the equations

The NKF laboratory implementation recommendations explain how to introduce and identify the new result. Repeated testing should make clear which method was used so that a formula change is not confused with a biological change.

Why removing race required a new fit

The older coefficient assigned different estimates according to a racial classification. The U.S. task force recommended a method that did not use race in the calculation. The 2021 equation has new coefficients fitted to population data, and no race input is required.

A race-free method still has estimation error and cannot infer an individual's muscle mass. Avoid treating the absence of a race variable as proof that every person's result is exact or that no population differences in performance remain.

What a switch can do to the result

At unchanged creatinine, age and sex, the 2021 and 2009 equations can give different values. Compared with the older race-adjusted equation, the new estimate is generally lower for people previously assigned the Black coefficient and often higher for those not assigned it. The size depends on the inputs; it is not a universal fixed correction.

NKF notes that some differences may exceed 10%, particularly at higher estimates and younger adult ages. Use the actual equations to compare an individual report rather than applying a percentage or interpreting the direction as kidney recovery or decline.

Accuracy is a separate question

NIDDK's adult equations reference emphasizes that these are estimates. It describes the combined creatinine-cystatin C equation as generally more accurate than creatinine alone. Even combined estimates have non-filtration influences and individual uncertainty.

No creatinine equation is the most accurate choice for every person or clinical situation. Muscle loss, very high muscle mass, pregnancy and rapidly changing kidney function need additional care. Drug dosing also requires attention to the particular label and units, as explained in the medication guide.

Compare reports without inventing a trend

1. Put creatinine values, units, dates and eGFR equations side by side.
2. Check whether the earlier report shows an exact value or only “≥60” or “≥90.”
3. Ask the laboratory or clinician about a formula change.
4. Consider urine albumin, symptoms, illness and medication changes alongside the blood results.
5. If the fall remains unexplained or is sudden, seek assessment rather than assuming the equation explains everything.

A difference in formula can explain part of a change, but real kidney changes can occur at the same time. The formula guide gives the CKD-EPI 2021 calculation and a checked example.