Estimated glomerular filtration rate using the 2021 CKD-EPI creatinine equation — the race-free version now recommended in the US and UK.
Glomerular filtration rate is the volume of blood your kidneys filter each minute, reported per 1.73 m² of body surface area so that results are comparable between people of different sizes. Measuring it directly requires infusing a marker substance and timed collections, which is impractical for routine care — so it is estimated from a blood creatinine result instead.
Creatinine is a waste product of muscle metabolism, produced at a fairly steady rate and cleared almost entirely by the kidneys. When filtration falls, creatinine accumulates. The equation works backwards from that concentration, adjusting for age and sex because both affect how much creatinine a person produces in the first place.
This calculator uses the 2021 CKD-EPI creatinine equation:
eGFR = 142 × min(Scr/κ, 1)α × max(Scr/κ, 1)−1.200 × 0.9938age × 1.012 [if female]
where κ is 0.7 for females and 0.9 for males, and α is −0.241 for females and −0.302 for males. Scr is serum creatinine in mg/dL.
The significant thing about the 2021 version is what it removed. Earlier equations, including the 2009 CKD-EPI, included a coefficient that raised the estimated GFR for Black patients by about 16%. That adjustment was based on observed average differences in creatinine generation, but race is a social category rather than a biological variable, and applying it meant two patients with identical bloodwork received different numbers — with the higher number potentially delaying referral, transplant listing and specialist care.
A joint National Kidney Foundation and American Society of Nephrology task force recommended removing it, and the race-free equation was published in 2021. It is now the recommended standard in the United States, and equivalent race-free reporting has been adopted in the UK.
| Category | eGFR | Description |
|---|---|---|
| G1 | 90 and above | Normal or high |
| G2 | 60–89 | Mildly decreased |
| G3a | 45–59 | Mildly to moderately decreased |
| G3b | 30–44 | Moderately to severely decreased |
| G4 | 15–29 | Severely decreased |
| G5 | Under 15 | Kidney failure |
Two points about this table are routinely misread. First, G1 and G2 are not chronic kidney disease on their own — a category only becomes CKD when there is also evidence of kidney damage, most often albumin in the urine. A perfectly healthy person sits in G1 or G2. Second, the diagnosis requires the abnormality to persist for three months or more. A single low reading, particularly during dehydration or acute illness, is not CKD.
Because creatinine comes from muscle, anything that changes muscle mass changes the estimate without anything happening to the kidneys.
High muscle mass — bodybuilders, some athletes — produces more creatinine, so eGFR is underestimated and kidney function looks worse than it is. Low muscle mass — frail elderly patients, amputees, people with cirrhosis or malnutrition — produces less, so eGFR is overestimated and genuine impairment can be missed. That second failure mode is the more dangerous one, because it hides a problem rather than inventing one.
Other things that shift the number: a large meal of cooked meat can raise creatinine for several hours; creatine supplements raise it; some drugs including trimethoprim and cimetidine block creatinine secretion in the tubules and raise the measured level without changing filtration at all.
Where the creatinine estimate is unreliable, cystatin C is the usual alternative. It is produced by all nucleated cells rather than muscle specifically, so it sidesteps the muscle-mass problem. Current guidance favours a combined creatinine-and-cystatin-C equation when a confirmatory estimate is needed.
Kidney disease is staged on two axes, not one: GFR category and albuminuria category. A person can have a completely normal eGFR and still have significant kidney disease evident as protein leaking into the urine — early diabetic kidney disease characteristically looks exactly like that. This is why a urine albumin-to-creatinine ratio is done alongside, and why a normal eGFR alone is not reassurance.
Above 90 is generally considered normal, and 60 to 89 is mildly decreased but often still normal for age — filtration declines gradually from around the fourth decade in most people. Neither category is chronic kidney disease by itself; CKD requires additional evidence of kidney damage, usually albumin in the urine, or an eGFR below 60 persisting for at least three months. Context matters more than the single number.
Because race is a social construct rather than a biological variable, and using it meant two patients with identical creatinine, age and sex received different estimated kidney function. The older coefficient raised eGFR by about 16% for Black patients, which could make kidney disease look less severe and delay referral, specialist care and transplant listing. A joint NKF and ASN task force recommended removal, and the 2021 race-free equation is now the recommended standard.
No. Chronic kidney disease requires a reduced eGFR that persists for at least three months. A single low reading can reflect dehydration, an acute illness, a recent high-protein meal, intense exercise, or certain medications rather than any lasting change in kidney function. The standard response to one unexpected result is to repeat it, not to act on it.
Because it is estimated from creatinine, which comes from muscle. Unusually high muscle mass produces more creatinine and makes kidney function look worse than it is. Unusually low muscle mass — frailty, amputation, cirrhosis, malnutrition — produces less and makes it look better, which can hide real impairment. Creatine supplements and drugs such as trimethoprim also raise creatinine without changing filtration. Where this is a concern, cystatin C gives an estimate that does not depend on muscle.
No. It is validated in adults aged 18 and over, and the calculator will not return a result below that age. Children need a paediatric equation — the CKiD U25 or a Schwartz-type formula — which accounts for the fact that creatinine production rises with growth. Using an adult equation on a child gives a badly wrong answer.
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