eGFR, the estimated glomerular filtration rate, is a calculated estimate of how much blood your kidneys filter each minute. The word that matters is estimated. An eGFR blood test measures exactly one thing, serum creatinine, and then runs that value through an equation along with your age and sex. Everything that makes creatinine an imperfect stand-in for kidney function therefore lands, unchanged, in the number you are looking at.
The Verdict
What an eGFR blood test actually measures
An eGFR blood test measures serum creatinine and calculates everything else. The glomeruli, the tiny filtering units in each kidney, cannot be watched at work in a routine panel, so the estimate works backwards from a substance the body produces steadily and the kidneys clear steadily. If clearance falls, that substance accumulates in the blood. Measure how much has accumulated, adjust for the fact that people of different ages and sexes produce different amounts, and you get a usable estimate of filtration.
Creatinine is the substance chosen for the job, mostly because it is cheap to measure and is on every metabolic panel already. The equation in widest use is the 2021 CKD-EPI creatinine equation, which takes creatinine, age and sex, and returns a result in millilitres per minute per 1.73 m² of body surface area. That last part is a normalisation for body size, and it means the reported figure is not your absolute filtration rate but your filtration rate scaled to a standard-sized adult.
The 2021 equation differs from its predecessors in one publicly discussed respect: it contains no race coefficient. Earlier equations, MDRD and the 2009 CKD-EPI, applied a multiplier that raised the reported eGFR for Black patients. Laboratories moved to the 2021 version at different times, so a change in your reported eGFR between two panels can reflect a change in the equation rather than a change in your kidneys. It is worth checking which one each lab used before reading a difference as a trend.
What creatinine is, and why it drives the whole calculation
Creatinine is a breakdown product of creatine and phosphocreatine in skeletal muscle, generated at a rate roughly proportional to how much muscle a person carries. It is not regulated, not recycled, and not useful to the body. It is filtered out by the glomeruli and excreted in urine, with a small additional amount secreted directly by the kidney tubules.
Read that description again with the equation in mind. The estimate assumes creatinine production is predictable from age and sex, so that a measured blood level can be attributed to how fast the kidney is clearing it. When production is not what the equation assumes, the estimate is wrong by the same amount, and nothing on the report says so. A very muscular 35-year-old man and a sedentary 35-year-old man of the same height receive the same assumed production rate. Their kidneys may be identical and their eGFR results will not be.
Standard eGFR ranges and what each band suggests
| Band | eGFR (mL/min/1.73 m²) | What it suggests |
|---|---|---|
| Normal or high | 90 or above | The band most healthy adults sit in. Kidney damage can still be present here, which is why urine albumin is checked separately. |
| Mildly reduced | 60–89 | Very common, and more common with age. Not chronic kidney disease on its own, in the absence of any evidence of kidney damage. |
| Mildly to moderately reduced | 45–59 | An eGFR below 60 sustained for three months or more is the conventional threshold that defines chronic kidney disease. Usually prompts a repeat draw and a urine albumin-to-creatinine ratio. |
| Moderately to severely reduced | 30–44 | Warrants medical follow-up, a medication review, and monitoring for the consequences of reduced filtration. |
| Severely reduced | Below 30 | Specialist kidney care. Below 15 is the band in which dialysis or transplantation is planned for. |
These bands map onto the conventional staging system for chronic kidney disease, where an eGFR under 60 sustained for at least three months, or evidence of kidney damage persisting for the same period, meets the definition. Staging is a clinical judgement made on repeat measurements alongside a urine test and your history. A single number from a home panel does not stage anyone, and reading it that way produces far more anxiety than information.
Low eGFR: what genuinely reduces kidney filtration
| Cause | How often it explains a result | Notes |
|---|---|---|
| Ageing | The single most common explanation for a result in the 60–89 band | Filtration falls gradually in most people from roughly the fourth decade onward. Expected is not the same as irrelevant, and it is also not a disease by itself. |
| Diabetes and high blood pressure | Together the leading causes of chronic kidney disease | Both damage the filtering units over years. Albumin usually appears in the urine before eGFR falls, which is why the urine test matters. |
| Dehydration or acute illness | Common, and usually temporary | Less blood reaching the kidney means less filtration for a few days. A repeat draw once well is what separates this from a lasting change. |
| Medications | Common, and frequently reversible | Regular NSAID use and several blood pressure drugs alter filtration pressure directly. Any decision about them belongs with the prescriber, not with the reader of a lab report. |
| Structural or immune kidney disease | Less common, and the reason a persistent fall gets investigated | Glomerulonephritis, polycystic kidney disease, and obstruction of urine flow. Identified with urine testing and imaging rather than with eGFR. |
Age deserves its position at the top of that table. Filtration declines gradually in most people across adult life, which is why the 60–89 band is thickly populated with people who have no kidney disease at all. That is also the clearest illustration of the site's standing position on normal versus optimal ranges: a reference range built from a population tells you what is common in that population, never what is good for you. An eGFR described as age-appropriate can still be falling faster than it should. The slope across several years carries more information than any single reading.
When the eGFR blood test misreads: creatinine's blind spots
| Situation | How often it matters | Effect on the calculated eGFR |
|---|---|---|
| High muscle mass | Common in this audience, and the classic false alarm | More muscle generates more creatinine at the same filtration rate, so the calculated eGFR reads lower than the true filtration rate. |
| Creatine supplementation | Common, and easily explained once declared | Creatine degrades to creatinine at a steady rate. Serum creatinine rises; filtration does not change. |
| A large cooked-meat meal shortly before the draw | Situational | Cooking converts some of the creatine in meat to creatinine, which is then absorbed. Enough to nudge a borderline result. |
| Drugs that block tubular secretion | Uncommon but well described | Trimethoprim and cimetidine raise serum creatinine by blocking its secretion into the urine, with no change in filtration at all. |
| Low muscle mass | The mirror image, and the more dangerous error | Frail, elderly, or limb-amputated people generate less creatinine, so eGFR is overestimated and reduced filtration can be missed. |
| Advanced liver disease | Uncommon, and a recognised pitfall | Reduced creatine production plus muscle wasting lowers serum creatinine, again flattering the calculated number. |
What eGFR means in combination
- Reduced eGFR + raised urine albumin-to-creatinine ratio: the combination that carries real prognostic weight. Filtration and damage are separate axes, and having both is different from having either.
- Normal eGFR + raised urine ACR: kidney damage with preserved filtration. This is the pattern a blood panel alone cannot see, and it is common early in diabetic kidney disease.
- Reduced eGFR + raised BUN, with a high BUN-to-creatinine ratio: points towards reduced blood flow to the kidney, from dehydration or heart failure, rather than damage to the kidney itself.
- Reduced eGFR + normal cystatin C: strongly suggests the creatinine, not the kidney, is the problem. This is the muscular or creatine-supplementing pattern.
- Reduced eGFR + low muscle mass or advanced liver disease: the estimate is probably generous rather than harsh, so a borderline result deserves more attention here, not less.
- A single mildly reduced eGFR with nothing else abnormal: most often a transient effect of dehydration, illness, or the day's food and training. Repeat it before treating it as a finding.
How and when to test
- eGFR and creatinine arrive on the standard metabolic panel. They sit alongside glucose, electrolytes and BUN on a comprehensive metabolic panel, so no separate order is usually needed. Our biomarker guides cover how the standard panel groups fit together.
- Be well hydrated and normally rested. Dehydration reduces filtration for a day or two and concentrates the sample, and both push the result the same way.
- Standardise what you do beforehand. Heavy resistance training and a large cooked-meat meal in the preceding day both nudge creatinine up. Keeping the conditions consistent between panels is what makes a year-on-year comparison meaningful.
- Declare creatine supplementation on the requisition, or tell the ordering clinician directly. It changes the interpretation of the result, and it is the single cheapest way to prevent an unnecessary investigation.
- Order a urine albumin-to-creatinine ratio alongside it, particularly with diabetes, high blood pressure, or a family history of kidney disease. eGFR alone cannot detect early damage.
- Repeat before reacting. A first mildly reduced result means very little on its own. The three-month interval in the chronic kidney disease definition exists precisely because single readings move around.
When a result warrants seeing a physician
- An eGFR below 60 that persists on a repeat test, which is the conventional threshold at which chronic kidney disease is considered and staged.
- Any eGFR below 45, or a sustained fall of roughly a quarter or more from your own established baseline, or a drop into a lower eGFR band, without an obvious explanation.
- A reduced eGFR alongside albumin or blood in the urine, swelling of the ankles or face, or a change in how much you are passing.
- A reduced eGFR in someone taking regular NSAIDs, lithium, or any medication cleared by the kidneys, since dose adjustment is a prescriber's decision and not a self-directed one.
- A creatinine that has climbed steadily across several panels while technically remaining inside the reference range.
- Any result you suspect is distorted by muscle mass or supplementation, since cystatin C is the test that resolves it and it needs to be ordered.
The eGFR blood test is a screening estimate with a known and specific weakness: it inherits every limitation of the creatinine it is built from. Treat a single mildly reduced value as a question rather than an answer. What it means for you belongs with a clinician who can see the urine result, the rest of the panel, your medication list, and how much muscle you are carrying.
Frequently Asked Questions
What is eGFR in a blood test?
eGFR stands for estimated glomerular filtration rate, an estimate of how many millilitres of blood your kidneys filter each minute, reported per 1.73 m² of body surface area. The word doing the work is estimated. Nothing about filtration is measured directly on a standard panel. The laboratory measures serum creatinine, then feeds that value along with your age and sex into an equation, and the equation returns the eGFR printed on your report. Understanding the test therefore means understanding creatinine, because every quirk of creatinine passes straight through into the number.
What is creatinine in a blood test?
Creatinine is a waste product formed when creatine and phosphocreatine in muscle break down, produced at a rate roughly proportional to how much muscle a person carries. It circulates in the blood, is filtered out by the kidneys, and leaves in the urine. Because production is fairly steady while removal depends on the kidneys, the blood level rises when filtration falls, which is what makes it useful. Most laboratories report something in the region of 0.7–1.3 mg/dL (about 60–115 µmol/L) for men and 0.6–1.1 mg/dL (about 55–95 µmol/L) for women, with real variation between labs and analysers.
What is a normal eGFR level?
An eGFR of 90 or above is conventionally described as normal, and 60–89 as mildly reduced. Many laboratories do not report an exact figure above 90 and simply print a greater-than sign, because the creatinine-based equations lose precision in the healthy range where they were least well validated. The more useful comparison is against your own previous results. A drop from 105 to 78 across three annual panels stays inside or near the normal band the whole way and still describes a real change worth explaining.
Does creatine supplementation raise creatinine and lower eGFR?
Yes, and it is one of the most common false alarms on an otherwise clean panel. Creatine degrades to creatinine at a steady rate, so supplementing it raises the serum creatinine that the eGFR equation is built on, without altering kidney filtration. On a report this shows up as a creatinine at or above the top of the range and an eGFR that has drifted down since the last panel. Two things prevent an unnecessary workup: telling whoever ordered the test that you supplement creatine, and cystatin C, an alternative filtration marker that does not depend on muscle or on creatine intake.
Why did my eGFR change when the lab updated its equation?
Reported eGFR can change because the calculation changed, not because your kidneys did. The 2021 CKD-EPI creatinine equation is now widely used and contains no race coefficient; earlier equations, including MDRD and the 2009 version of CKD-EPI, applied a multiplier for Black patients that produced a higher reported eGFR. Laboratories adopted the 2021 equation at different times. If a result shifted between two panels with no clinical change, it is worth checking which equation each lab used before treating the difference as a trend.
Does eGFR fall with age?
Yes. Filtration declines gradually in most people from around the fourth decade onward, which is why a large share of otherwise healthy older adults sit in the 60–89 band. This is exactly where a population reference range stops being a health target: the range describes what is common at a given age, not what is desirable. Two people can both be told their eGFR is age-appropriate while one has been stable for a decade and the other has lost 20 points in three years. The trajectory carries the information, not the single value.
Can I have kidney damage with a normal eGFR?
Yes, and this is the biggest blind spot of the test. Kidney damage often shows up first as albumin leaking into the urine, while filtration is still fully preserved. That is what the urine albumin-to-creatinine ratio detects, and it is a separate test from anything on a blood panel. Chronic kidney disease is defined by reduced filtration or evidence of kidney damage persisting for three months or more, so an eGFR of 95 with persistent albuminuria still meets the definition. Running eGFR without a urine ACR answers only half the question.
How is eGFR different from creatinine clearance?
eGFR comes from an equation applied to a single blood sample; creatinine clearance comes from an actual timed urine collection, usually over 24 hours, measured alongside a blood creatinine. The urine collection avoids the age and sex assumptions baked into the equation, but it is inconvenient and frequently collected incorrectly, which is why estimating equations replaced it for routine use. Neither is a true measured GFR. That requires an injected filtration marker such as iohexol, which is a research and transplant-assessment procedure rather than a screening test.