Most people meet their kidneys through a single line on a routine blood panel: eGFR. It sits next to sodium and glucose on a basic metabolic panel, usually followed by a number over 90 and no further comment. Almost nobody stops to notice the word doing the real work in that abbreviation: estimated.
eGFR is not a direct measurement of how your kidneys filter blood. It is a calculation, built from a blood test and a formula, and in September 2021 that formula was rewritten for the entire country. What actually changed, and why, says more about how kidney function is read than any single lab result.
What the number is trying to capture
The kidneys filter blood through roughly a million tiny clusters of vessels apiece, called glomeruli. The rate at which they do this, in milliliters of blood cleared per minute, is the glomerular filtration rate, or GFR. The National Kidney Foundation estimates that 37 million U.S. adults may have chronic kidney disease, and nearly 90 percent do not know it, largely because early-stage disease produces no symptoms.
Measuring GFR directly is possible but impractical, requiring an injected filtration marker and hours of tracked clearance, a process reserved mostly for research and pre-transplant evaluation, according to Cleveland Clinic. For everyday care, providers estimate it instead, from a blood test that stands in for the real thing: creatinine.
Why creatinine is the stand-in
Creatinine is a waste product of creatine, a compound your muscles use for energy. As muscle tissue does its ordinary work, some creatine breaks down into creatinine, which enters the blood and is normally cleared out by the kidneys and released in urine, per MedlinePlus. When the kidneys filter less effectively, creatinine has nowhere to go, and it accumulates in the blood.
That relationship makes creatinine a workable proxy for filtration, but an imperfect one. Because it is a byproduct of muscle metabolism, the amount your body produces daily depends on how much muscle you carry, not only how well your kidneys work. A bodybuilder and a frail older adult can generate very different amounts of creatinine with identical kidney function, and the American Kidney Fund notes that pregnancy, being under 18, and unusual muscle mass or body composition can all shift the reading independent of the kidneys. A large recent meal of cooked meat can nudge it up too, per MedlinePlus, which is why some labs ask patients to avoid meat before the draw.
To correct for this, the eGFR formula never used creatinine alone. It combines the creatinine result with age and sex, since both shift muscle mass and creatinine production in fairly predictable ways. For decades, in the United States, it also used a coefficient for Black race.
The variable that got removed
The race coefficient entered the standard eGFR equation because population studies from the late 1990s found that, on average, Black patients in those samples had higher measured creatinine clearance at a given true GFR than white patients did, and researchers built that average difference into the formula as a multiplier. In practice, the coefficient reported a higher eGFR for a Black patient than a white patient with the same creatinine, age, and sex.
The problem, as a joint task force of the National Kidney Foundation and the American Society of Nephrology concluded, is that race is a social category, not a biological measurement, and folding it into a filtration equation risked masking real kidney disease in Black patients by reporting an artificially reassuring number. Convened in July 2020 to review the evidence, the task force wrote in a March 2021 joint statement that race modifiers should not be included in equations used to estimate kidney function. That September, it released its final recommendation: adopt the 2021 CKD-EPI creatinine equation, which drops the race variable entirely.
The change was not cosmetic. The American Journal of Kidney Diseases published a position statement from the National Kidney Foundation's Kidney Disease Outcomes Quality Initiative endorsing the immediate switch, noting the new equation could remove barriers for some Black patients to a timely CKD diagnosis, an earlier nephrology referral, and earlier transplant waitlist access. Most major U.S. labs have since adopted the race-free formula, per the American Kidney Fund, though the same creatinine value may now report a somewhat lower eGFR than under the old equation.
Reading the actual number
Once calculated, eGFR is reported as a rate, in milliliters per minute per 1.73 square meters of body surface area, normalized so results are comparable across differently sized bodies. Cleveland Clinic lists a normal adult eGFR as roughly 90 or above, declining gradually with age even in healthy kidneys, from an average of about 116 in the 20s to closer to 75 by 70 and beyond. A result under 60 that persists for three months or more generally defines chronic kidney disease, staged from mild (60 to 89, with other evidence of damage) down to kidney failure below 15.
That said, one reading rarely settles anything. MedlinePlus is direct about this: creatinine testing alone is not the best way to check kidney function, and levels can stay in a nominally normal range through the early stages of real kidney disease, only rising once damage is more advanced. A single abnormal result is typically followed by a repeat test, a urine albumin test, or both, rather than an immediate diagnosis.
The second opinion: cystatin C
Because creatinine's dependence on muscle mass is its clearest weak spot, an alternative marker sidesteps the problem. Cystatin C is a small protein produced at a fairly constant rate by essentially all the body's cells, not concentrated in muscle, so it is far less sensitive to age, muscle mass, or diet, according to Cleveland Clinic. It is particularly useful for people over 65 or with unusually high or low muscle mass, where a creatinine-only eGFR is more likely to mislead.
The 2021 task force did not stop at removing race from the creatinine equation. It also recommended wider, more routine use of cystatin C, ideally combined with creatinine, as a confirmatory check on eGFR for patients at risk of kidney disease or already diagnosed with it. Combining two independent filtration markers reduces the error either one carries alone. Cystatin C testing has been slower to reach routine panels than the equation update, but its use is expanding.
Common questions
Is a low eGFR always kidney disease?
Not on its own. A single reading below 90 can reflect normal age-related decline, dehydration, recent heavy exercise, or another temporary factor. Cleveland Clinic notes a diagnosis generally requires an eGFR under 60 sustained for three months or more, confirmed with a repeat test or a urine albumin test.
Why does eGFR decline with age even in healthy people?
Kidney filtration capacity gradually decreases as part of normal aging, independent of disease, which is why Cleveland Clinic's reference table shows a falling average eGFR by decade, from around 116 in the 20s to roughly 75 at 70 and older. That is one reason a fixed cutoff is read differently depending on a patient's age.
Does a high-protein diet or exercise affect my result?
Yes, temporarily. MedlinePlus notes a large serving of cooked meat the day or two before a blood draw can raise creatinine, and intense recent exercise can do the same, without any real change in kidney function. That is part of why an isolated abnormal result is usually retested rather than acted on immediately.
What changed for patients after the 2021 equation update?
For most people, the race-free CKD-EPI equation produces a broadly similar result to the old formula. For some Black patients, the National Kidney Foundation's Kidney Disease Outcomes Quality Initiative statement notes the update can report a somewhat lower eGFR than the earlier race-adjusted formula would have, which in some cases has meaningfully changed the timing of a CKD diagnosis, a nephrology referral, or transplant waitlist eligibility.
Should I ask for a cystatin C test?
It is worth raising if your creatinine-based eGFR seems inconsistent with how you feel, or if you fall into a group where creatinine runs less reliable, such as older adults or people with unusually high or low muscle mass. Cleveland Clinic notes cystatin C is not affected by age, muscle mass, or diet the way creatinine is, making it a useful second opinion rather than a routine replacement.
