Estimated GFR

What is eGFR?

Estimated glomerular filtration rate (eGFR) is a calculated number, not something measured in the blood. Glomerular filtration rate itself — the volume of blood plasma the kidneys' filtering units clear each minute, expressed per 1.73 m² of body surface — can only be measured directly by infusing a tracer and timing its disappearance, which is impractical for routine care. eGFR approximates it by taking serum creatinine and adjusting for the two things that most change how much creatinine a person produces: age and sex. Laboratories report it automatically with every creatinine result, and it is the value on which chronic kidney disease is defined and staged.

How is eGFR calculated?

eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^Age × 1.012 [if female]; Scr in mg/dL; κ = 0.7 (female) / 0.9 (male); α = −0.241 (female) / −0.302 (male)

Inputs: CR · age · sex

This is the CKD-EPI 2021 creatinine equation, recommended by KDIGO 2024 and adopted by most laboratories. It is an estimate: for any individual the true GFR lies within about ±30% of the reported value nine times out of ten, and precision is poorest above 60. Companion equations exist that use cystatin C instead of creatinine, or both together; the combined creatinine-cystatin C version is the most accurate and is recommended whenever the creatinine result may be distorted by muscle mass or medication. Earlier equations (MDRD, CKD-EPI 2009) multiplied the result by a factor for people identified as Black; the 2021 equation removed that coefficient because race is a social rather than biological category, the adjustment could not be applied consistently, and it delayed diagnosis and transplant listing for Black patients. Some laboratories still report the 2009 version, so the equation used should be checked when comparing results.

What does eGFR do in the body?

eGFR reflects how much filtering work the kidneys are doing, which is the single most useful summary of kidney function. Each kidney contains about a million glomeruli, tiny capillary tufts that push water and small molecules out of the blood into the tubules while retaining cells and proteins. A young adult filters roughly 90-120 mL of plasma per minute per 1.73 m² — about 180 litres a day, nearly all of it reabsorbed. Filtration declines slowly with age even in health, by around 1 mL/min/1.73 m² a year after the fourth decade, and faster with diabetes, high blood pressure or glomerular disease. Because a kidney with fewer working glomeruli compensates by making the remaining ones work harder, eGFR can stay stable for years before a decline becomes visible — which is why it is read together with urine albumin.

What does a high eGFR mean?

A high eGFR is usually of no concern and most laboratories simply report it as above 90 or above 60 without a number. Two situations give it some meaning. First, a falsely high estimate: anything that lowers creatinine for non-kidney reasons — little muscle, pregnancy, liver disease, a vegan diet — makes the equation overstate filtration, and cystatin C gives the correction. Second, genuine hyperfiltration: in early diabetes, obesity and pregnancy the kidneys filter more than normal, and in diabetes this phase has been associated in cohort studies with faster later decline. Above about 60 the equation is too imprecise for small differences to carry any message.

Common causes:

• Low muscle mass, which lowers creatinine and inflates the estimate — the most common explanation in older or underweight people.
• Pregnancy, in which true filtration rises by up to 50%.
• Early diabetes or obesity, in which the kidneys hyperfilter before any damage shows.
• A high-protein diet, which raises filtration modestly over the following hours.

What does a low eGFR mean?

A low eGFR means the kidneys are estimated to be filtering less than expected for the person's age and sex. KDIGO 2024 defines chronic kidney disease as an eGFR below 60 mL/min/1.73 m², or other evidence of kidney damage such as albumin in the urine, persisting for more than three months, and grades it in categories from G1 (90 or above) through G3a (45-59), G3b (30-44) and G4 (15-29) to G5 (below 15). A single low value is not a diagnosis: it has to be confirmed over time, and the confounders that raise creatinine without touching the kidneys — muscle, creatine, a meat meal, certain drugs — have to be excluded, ideally with a cystatin C-based estimate.

Common causes:

• Chronic kidney disease, most often from diabetes or high blood pressure, less often from glomerulonephritis, polycystic kidney disease or long-term urinary obstruction.
• Acute kidney injury — dehydration, severe infection, a urinary blockage, or a drug such as an NSAID or contrast agent — which typically recovers.
• High muscle mass, creatine supplements or a recent cooked-meat meal, none of which affect the kidneys but all of which raise creatinine and pull the estimate down.
• Medications that block creatinine secretion — trimethoprim, cimetidine, fenofibrate, dolutegravir, cobicistat — producing an apparent fall of 10-30% with no true change.
• Normal ageing: a healthy 80-year-old commonly has an eGFR in the 60s, which the equation partly but not fully allows for.

What moves eGFR apart from disease?

Muscle mass relative to age and sex either direction

The equation assumes average muscle for a person's age and sex, because that is what its creatinine term stands in for. Anyone far from that average — a bodybuilder, a marathon runner, a frail older adult, an amputee, a person with a muscle disease — gets a systematically wrong estimate in a predictable direction: more muscle, lower eGFR; less muscle, higher eGFR.

The age and sex inputs themselves either direction

eGFR falls by design as age increases even when creatinine is unchanged, because the equation expects older people to make less creatinine. The same creatinine value therefore yields a lower eGFR for a woman than a man, and for a 70-year-old than a 30-year-old. A mis-entered birth date or sex changes the result outright.

Creatine supplements lowers it

Creatine monohydrate raises serum creatinine for as long as it is taken and so lowers the calculated eGFR, without any change in true filtration. A cystatin C-based estimate is unaffected and is the straightforward way to resolve the discrepancy.

A cooked-meat meal before the draw lowers it

Cooked meat delivers pre-formed creatinine that is absorbed within hours and can push a borderline eGFR across a staging boundary. Overnight fasting removes the effect.

Hydration either direction

Dehydration reduces kidney blood flow, raises creatinine and lowers eGFR temporarily; this is a real but reversible change rather than kidney damage. Urea usually rises out of proportion at the same time, which is the clue.

Drugs that block tubular creatinine secretion lowers it

Trimethoprim, cimetidine, fenofibrate, dolutegravir and cobicistat each raise creatinine by competing for the same transporter, lowering eGFR by 10-30% within days of starting and restoring it when stopped. NSAIDs, ACE inhibitors, ARBs and SGLT2 inhibitors are different: they lower true filtration slightly, and for the latter three that small initial dip is expected and protective in the long run.

Which other tests are read together with eGFR?

Creatinine upstream

Serum creatinine is the measured input from which eGFR is computed; every confounder of creatinine is therefore a confounder of eGFR.

Cystatin C same panel

Cystatin C feeds the alternative and the combined equations. KDIGO 2024 recommends a cystatin C-based estimate to confirm CKD when the creatinine-based eGFR is 45-59 with no other kidney damage marker, and whenever muscle mass or medication makes creatinine unreliable.

Blood Urea Nitrogen same panel

Urea rises with falling filtration too, but also with dehydration, bleeding into the gut and high protein intake. A urea rise out of proportion to the eGFR fall points to one of those rather than to kidney disease.

Uric Acid downstream

Uric acid is cleared by the kidney, so it rises as eGFR falls, and hyperuricaemia is in turn associated with faster progression of kidney disease.

Systolic Blood Pressure upstream

High blood pressure is among the two leading causes of kidney disease, and its control is one of the main levers for slowing eGFR decline.

Glycated Hemoglobin (HbA1c) upstream

Diabetes is the most common cause of kidney failure. Long-term glucose control strongly predicts the rate at which eGFR falls, and KDIGO 2024 recommends annual eGFR and urine albumin checks in everyone with diabetes.

Which foods affect eGFR?

• Salt below 5 g a day (about 2 g sodium), the KDIGO 2024 target for people with kidney disease — supports strong evidence
• Plant-forward patterns such as DASH or Mediterranean, associated with slower loss of filtration in cohorts — supports moderate evidence
• Protein intake above about 1.3 g/kg a day, which KDIGO 2024 advises against in people at risk of progression — lowers moderate evidence
• A large cooked-meat meal in the hours before testing, which lowers the estimate without touching the kidneys — lowers strong evidence

Which supplements are studied for eGFR?

Creatine monohydrate strong evidence

Creatine is taken for strength and muscle, not for the kidneys, but it is the supplement most likely to produce an alarming eGFR: the extra creatine converts to creatinine at a steady rate, and the equation reads the higher creatinine as lower filtration. Studies in healthy adults at these doses have not shown harm to true kidney function.
Typical dose: 3-5 g/day, the usual sports-nutrition maintenance dose
Cautions: Tell the clinician ordering the test that creatine is being taken, and ask for a cystatin C-based estimate if the eGFR looks low. Safety has not been established in people who already have kidney disease, and it is usually advised against in that group.

How does exercise affect eGFR?

Regular moderate aerobic activity strong evidence

How much: At least 150 minutes a week of moderate intensity, the level KDIGO 2024 recommends for people with kidney disease as for everyone else.
What to expect: Lowers blood pressure, improves insulin sensitivity and cardiovascular fitness — the main determinants of how fast eGFR declines over the years. Trials in kidney disease show better physical function and quality of life; the effect on eGFR itself is small and slow.

Resistance training moderate evidence

How much: Two or three sessions a week covering the major muscle groups.
What to expect: Gaining muscle raises creatinine and so lowers the calculated eGFR slightly and permanently — a healthy change that the equation misreads, not a kidney signal. A hard, unaccustomed session also lowers eGFR for a day or two, which is why heavy training is kept away from blood draws.

Which lifestyle factors affect eGFR?

Blood pressure and glucose control strong evidence

These two, more than anything else, determine the slope of eGFR over a decade. Sustained control of blood pressure and, in diabetes, of glucose is associated with a markedly slower decline and a lower chance of ever needing dialysis.

Avoiding regular NSAID use strong evidence

Ibuprofen, naproxen, diclofenac and related drugs reduce kidney blood flow and, taken regularly, are associated with faster loss of filtration — especially with dehydration, diuretics or blood-pressure medication. Paracetamol at normal doses does not share this effect.

Smoking cessation strong evidence

Smoking is an independent risk factor for developing kidney disease and for faster progression once it exists; stopping is part of routine kidney care in KDIGO 2024.

Reading eGFR with urine albumin strong evidence

Kidney disease is graded on two axes — eGFR and the urine albumin-to-creatinine ratio — and a normal eGFR with albumin in the urine still counts as kidney disease. Anyone with diabetes, high blood pressure or a family history benefits from having both checked, not eGFR alone.

How should you prepare for a eGFR test?

• Fast overnight, or at least avoid a large meat meal for 12 hours, so that dietary creatinine does not pull the estimate down.
• Skip strenuous or unaccustomed exercise for 24-48 hours beforehand and drink normally — dehydration lowers eGFR temporarily.
• Mention creatine supplements and all medications, especially trimethoprim, cimetidine, fenofibrate and HIV drugs, which lower eGFR without affecting the kidneys.
• A single eGFR below 60 needs confirming: KDIGO 2024 requires the finding to persist for more than three months before chronic kidney disease is diagnosed, and recommends a cystatin C-based estimate when the value is 45-59 with no other sign of kidney damage.
• Check which equation the laboratory uses. Results from the 2009 and 2021 CKD-EPI equations differ by a few units, and the 2009 version applied a race coefficient that the 2021 version does not.
• Where muscle mass is far from average — very athletic, very frail, an amputation, a muscle disease — a cystatin C estimate is the more trustworthy number.

References

1. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int 2024 — https://doi.org/10.1016/j.kint.2023.10.018
2. Inker LA et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med 2021 — https://doi.org/10.1056/NEJMoa2102953
3. Delgado C et al. A Unifying Approach for GFR Estimation: Recommendations of the NKF-ASN Task Force on Reassessing the Inclusion of Race in Diagnosing Kidney Disease. Am J Kidney Dis 2022 — https://doi.org/10.1053/j.ajkd.2021.08.003
4. Levey AS et al. A New Equation to Estimate Glomerular Filtration Rate (CKD-EPI 2009). Ann Intern Med 2009 — https://doi.org/10.7326/0003-4819-150-9-200905050-00006
5. Levey AS, Inker LA. Assessment of Glomerular Filtration Rate in Health and Disease: A State of the Art Review. Clin Pharmacol Ther 2017 — https://doi.org/10.1002/cpt.729