eGFR Calculator (CKD-EPI 2021)

Glomerular filtration rate is the volume of blood the kidneys clear of waste each minute. It cannot be measured directly in routine practice, so it is estimated from creatinine — a breakdown product of muscle that healthy kidneys clear efficiently. As filtration falls, creatinine rises, and the equation turns that relationship into a filtration rate.

A young adult with healthy kidneys typically filters somewhere around 100 to 130 mL/min/1.73 m². The result is indexed to 1.73 m² of body surface area — a standard adult size — so that a small person and a large person can be compared on the same scale. That indexing is also why the number should not be used directly for weight-based drug dosing without further thought.

Above roughly 60, the estimate becomes imprecise: creatinine barely moves across a wide span of true filtration, so a reported 95 and a reported 110 may not be meaningfully different. Below 60, the estimate tightens considerably, which is one reason the CKD categories are drawn where they are.

Older equations included a multiplier applied to anyone recorded as Black. MDRD raised the result by about 21% and CKD-EPI 2009 by about 16%, on the observation that average creatinine differed between the study populations. The problem is what that assumption does at the bedside: identical bloodwork produced a higher reported kidney function for Black patients, which pushes them later past the thresholds used for nephrology referral, medication adjustment, and transplant waiting-list eligibility.

Race is a social category, not a biological measure of muscle mass or creatinine generation, and it is frequently recorded inconsistently or self-reported. In 2021 a joint task force of the National Kidney Foundation and the American Society of Nephrology recommended that race be removed from GFR estimation. The CKD-EPI 2021 creatinine equation was developed and validated for exactly that purpose.

The practical consequence is that this calculator will disagree with an older result on the same blood sample for some patients. If you are comparing a new eGFR against one from before 2022, check which equation the laboratory used — the change in reported value can look like a change in kidney function when nothing has changed at all.

KDIGO sorts filtration into six categories, G1 through G5. These describe the GFR only — they are not, on their own, a diagnosis of chronic kidney disease.

Chronic kidney disease requires either a GFR below 60 or a marker of kidney damage, present for more than three months. That is why G1 and G2 exist at all: someone with an eGFR of 95 and persistent albuminuria has CKD, while someone with an eGFR of 75 and a clean urine test usually does not.

Full KDIGO staging pairs the G category with an albuminuria category — A1 (under 30 mg/g), A2 (30–300 mg/g), or A3 (over 300 mg/g) from a urine albumin-to-creatinine ratio. Prognosis depends on both. An eGFR of 50 with A1 albuminuria carries substantially less risk than the same eGFR with A3.

Creatinine comes from muscle, so anything that changes muscle mass or creatinine handling shifts the estimate without any change in filtration:

• Very muscular people generate more creatinine, so their eGFR reads falsely low. Amputation, paralysis, frailty, and advanced liver disease do the reverse.
• A large meat meal or creatine supplementation raises creatinine for a day or so.
• Trimethoprim, cimetidine, and some other drugs block creatinine secretion in the tubule — creatinine rises and eGFR falls without any change in true filtration.
• The equation assumes a steady state. In acute kidney injury, creatinine lags behind the real filtration rate by a day or more, so eGFR overestimates function while injury is developing.
• It is not validated in pregnancy, in children, or in people under 18.

A single result is also a snapshot. Chronic kidney disease is defined by abnormality persisting beyond three months, so an isolated low reading warrants a repeat before it means anything.

Measured GFR uses an exogenous filtration marker such as iohexol or iothalamate. It is the reference standard, and it is slow and expensive enough that it is reserved for research and for cases such as living kidney donation.

Creatinine clearance from the Cockcroft-Gault equation is a different quantity: it estimates clearance in mL/min using actual body weight, and it is not indexed to body surface area. Several drug labels still specify Cockcroft-Gault for dose adjustment, so the two are not interchangeable when dosing renally cleared medication.

A first step is confirmation: repeat the creatinine, and add a urine albumin-to-creatinine ratio, because albuminuria carries as much prognostic weight as the filtration rate itself. Look at the trajectory too — an eGFR that has fallen from 80 to 55 over two years is a different problem from one that has sat at 55 for a decade.

KDIGO suggests nephrology referral for an eGFR below 30, a sustained fall of 25% or more, albuminuria above 300 mg/g, unexplained haematuria, or CKD with uncontrolled hypertension. Much of the management at earlier stages — blood pressure control, SGLT2 inhibitors, avoiding nephrotoxins including routine NSAIDs — happens in primary care.