Estimated Glomerular Filtration Rate Blood Test
What It Is, Why It Matters, and How to Interpret Your Results
Estimated glomerular filtration rate, or eGFR, provides an estimate of how efficiently the kidneys filter waste and excess fluid from the blood. It is calculated from creatinine, age, and sex rather than measured directly.
When interpreted in context, eGFR helps identify changes in kidney filtration and provides insight into how well the kidneys are supporting fluid balance, electrolyte regulation, blood pressure control, and metabolic waste removal. It is most informative when interpreted alongside creatinine, urine albumin, relevant health history, and trends over time.
Quick Take
eGFR estimates how much blood the kidneys filter each minute, adjusted to a standard body surface area. It is reported in milliliters per minute per 1.73 square meters, written as mL/min/1.73 m².
Rythm calculates eGFR from creatinine, age, and sex using the 2021 CKD-EPI creatinine equation. This modern equation does not include race.
Rythm displays values from 90 to 140 as optimal, values from 60 to 89 as normal, values below 60 as out of range, and values below 15 as critical. However, eGFR is an estimate rather than a direct measurement, and one result cannot diagnose chronic kidney disease. Age, muscle mass, hydration, illness, medications, and other markers of kidney health all affect interpretation.
Why Strive for Optimal eGFR?
Healthy kidney filtration allows the body to remove metabolic waste while maintaining appropriate fluid, electrolyte, and acid-base balance. The kidneys also contribute to blood pressure regulation, red blood cell production, and mineral metabolism.
The goal is not to push eGFR as high as possible. A higher result within the expected range does not necessarily indicate better health, and values at the upper end of the scale are estimated less precisely.
Instead, the goal is to preserve stable kidney filtration over time. A consistent eGFR within the expected range generally supports healthy kidney function, while a sustained decline can signal reduced filtration and increased long-term health risk.
What Does Optimal eGFR Mean?
Rythm displays an eGFR from 90 to 140 mL/min/1.73 m² as optimal. This range includes the filtration levels generally expected in healthy younger and middle-aged adults.
This does not mean that 140 is better than 100 or that an isolated result slightly above 140 is necessarily abnormal. At higher filtration levels, creatinine-based equations become less precise. Unusually high estimates can also be influenced by low muscle mass, pregnancy, body size, or physiological hyperfiltration.
Rythm displays values from 60 to 89 as normal. Clinically, this range is described as mildly lower than young-adult levels. It is common with increasing age and does not establish kidney disease by itself, particularly when there are no other markers of kidney damage.
Values below 60 are displayed as out of range because they fall below the filtration level generally expected in adults and deserve additional clinical context. Whether a low result reflects temporary change, acute kidney stress, or chronic disease depends on repeat testing, duration, urine albumin, medical history, and other findings.
Why Tracking eGFR Over Time Matters
eGFR is more informative as a trend than as a single result.
Creatinine and eGFR can change temporarily during dehydration, acute illness, medication changes, or other disruptions in normal physiology. Small shifts may also reflect normal biological or analytical variation.
Tracking eGFR over time helps distinguish a temporary fluctuation from a sustained decline in kidney filtration. Comparisons are most meaningful when the same equation and creatinine method are used consistently, since changing equations can create an apparent change even when kidney function has remained stable.
Chronic kidney disease generally requires evidence of abnormal kidney structure or function lasting at least three months. However, a sudden or substantial decrease in eGFR should not simply be ignored until three months have passed, since it may reflect acute kidney injury requiring more immediate evaluation.
What Is eGFR?
The kidneys contain millions of microscopic filtering units called glomeruli. These structures filter water and small molecules from the blood while normally retaining blood cells and most proteins.
Glomerular filtration rate describes the volume of fluid filtered by the glomeruli over time. Measuring GFR directly requires specialized procedures that are not practical for routine testing.
Instead, laboratories estimate GFR using a filtration marker such as creatinine. The resulting eGFR is normalized to a standard adult body surface area of 1.73 m² so results can be compared more consistently across individuals.
Why eGFR Matters
A broad indicator of kidney filtration
eGFR translates creatinine into an estimate of kidney filtration that accounts for age and sex. This makes it more informative than interpreting creatinine in isolation.
A screening and monitoring marker
eGFR can help identify reduced kidney filtration before obvious symptoms develop. It is also used to monitor kidney function over time and provide context for medication dosing and other clinical decisions.
Not a standalone diagnosis
eGFR does not identify the cause of reduced filtration and cannot fully assess kidney health by itself. Kidney disease may be present even when eGFR is above 60 if there are other markers of kidney damage, such as elevated urine albumin.
Conversely, an eGFR between 60 and 89 does not establish chronic kidney disease when there is no albuminuria, structural abnormality, or other evidence of kidney damage.
eGFR Variability and Physiological Factors
Because eGFR is calculated from creatinine, anything that changes creatinine can also change the estimated filtration rate.
People with greater muscle mass may naturally produce more creatinine, which can make creatinine-based eGFR appear lower even when kidney function is healthy. People with low muscle mass, frailty, an amputation, or muscle-wasting conditions may produce less creatinine, potentially making eGFR appear higher than their actual filtration rate.
Dietary patterns, body size, acute illness, pregnancy, and certain medications can also affect creatinine or the accuracy of the equation. Creatinine-based eGFR is least reliable when kidney function or creatinine is changing rapidly.
These limitations do not make eGFR unhelpful. They mean that the estimate should be interpreted as part of a broader pattern rather than treated as an exact measurement.
Who Should Pay Extra Attention to eGFR?
eGFR is especially important to monitor in individuals with diabetes, high blood pressure, cardiovascular disease, a family history of kidney disease, prior acute kidney injury, recurrent kidney stones, or known structural kidney abnormalities.
It may also deserve closer attention in people taking medications that can affect kidney function, older adults, and individuals with a sustained rise in creatinine.
People with unusually high or low muscle mass should interpret creatinine-based eGFR cautiously. When greater precision is needed, an estimate incorporating cystatin C may provide additional information because it is affected by different non-kidney factors than creatinine.
How eGFR Is Calculated
Rythm calculates eGFR using the race-free 2021 CKD-EPI creatinine equation. The calculation uses creatinine in milligrams per deciliter, age in years, and sex.
For females:
eGFR = 142 × min(creatinine/0.7, 1)^−0.241 × max(creatinine/0.7, 1)^−1.200 × 0.9938^Age × 1.012
For males:
eGFR = 142 × min(creatinine/0.9, 1)^−0.302 × max(creatinine/0.9, 1)^−1.200 × 0.9938^Age
In these equations, “min” selects the lower value between the creatinine ratio and 1, while “max” selects the higher value.
The equation adjusts for the average relationships between creatinine production, age, sex, and kidney filtration observed across large populations. However, those adjustments cannot account perfectly for each individual’s muscle mass, diet, body composition, or health status.
Testing Considerations
eGFR does not require a separate assay. It is calculated automatically whenever the required creatinine, age, and sex information is available.
Testing under reasonably consistent conditions improves comparison over time. Acute illness, dehydration, major changes in diet or supplementation, and recent medication changes may alter creatinine or kidney filtration.
The 2021 CKD-EPI equation is intended for adults. Pediatric equations should be used for children and adolescents.
eGFR should also not be used to assess kidney function during pregnancy. Pregnancy produces substantial physiological changes in filtration and creatinine, and standard eGFR equations have not been validated for this setting. Kidney function during pregnancy should be evaluated using pregnancy-appropriate clinical assessment and direct interpretation of creatinine.
What eGFR Levels Mean
90–140 mL/min/1.73 m²: Optimal
This is the optimal range displayed by Rythm and generally represents expected kidney filtration in adults.
A value within this range does not need to be pushed higher. Differences within the range may reflect age, body composition, creatinine production, and normal variation rather than meaningful differences in kidney health. Values above 140 should not automatically be viewed as superior kidney function. Creatinine-based equations are less precise at high filtration levels, and a very high estimate may reflect low creatinine production, pregnancy, body composition, or hyperfiltration rather than exceptionally healthy kidneys.
60–89 mL/min/1.73 m²: Normal
Rythm displays this range as normal. It represents filtration that is mildly lower than typical young-adult levels.
This range becomes increasingly common with age and does not indicate chronic kidney disease by itself. Other findings, particularly urine albumin and trends over time, determine whether it is clinically significant.
45–59 mL/min/1.73 m²: Below Expected Range
This range reflects a meaningful reduction in estimated filtration. When persistent for at least three months, it corresponds to the G3a kidney function category.
A single result should be repeated and interpreted with creatinine history, urine albumin, health conditions, medications, and recent illness.
30–44 mL/min/1.73 m²: Low
This range reflects moderately to severely reduced filtration and corresponds to the G3b category when persistent.
Clinical evaluation is important because reduced filtration at this level may affect medication handling and long-term kidney and cardiovascular risk.
15–29 mL/min/1.73 m²: Very Low
This range reflects severely reduced filtration and corresponds to the G4 category when persistent.
Results in this range require timely medical evaluation and ongoing clinical management.
Below 15 mL/min/1.73 m²: Critically Low
An eGFR below 15 is critically low and falls within the G5 category.
A single result cannot determine whether the cause is acute or chronic, but a result at this level requires prompt medical assessment.
Factors That Influence eGFR
Kidney filtration
A decrease in kidney filtration generally causes creatinine to rise and eGFR to fall.
Muscle mass and body composition
Greater muscle mass can increase creatinine production and lower calculated eGFR. Low muscle mass can reduce creatinine and make calculated eGFR appear higher.
Age
The equation incorporates age because expected filtration and creatinine production change across adulthood. A result from 60 to 89 may be more concerning in a young adult than in an older adult, depending on the broader clinical context.
Hydration and acute illness
Dehydration, reduced kidney blood flow, infection, and acute illness can temporarily raise creatinine and lower eGFR.
Diet, supplements, and medications
Dietary patterns, creatine use, and certain medications can affect creatinine concentration or kidney function. These factors should be considered when a result changes unexpectedly.
Pregnancy
Pregnancy increases kidney filtration and changes creatinine physiology. Standard eGFR equations are not valid during pregnancy and should not be used to classify kidney function in that setting.
How eGFR Fits With Other Rythm Biomarkers
eGFR is most directly connected to creatinine because creatinine is the laboratory measurement used in the calculation. When creatinine rises, eGFR generally falls. When creatinine falls, eGFR generally rises.
Uric acid can provide additional context because the kidneys contribute to uric acid clearance, although uric acid is also strongly influenced by diet, metabolism, medications, and genetics.
Fructosamine provides context around recent glycemic exposure, while ApoB and hsCRP help characterize cardiometabolic and inflammatory risk that may overlap with long-term kidney risk.
Serum albumin provides information about circulating albumin but should not be confused with urine albumin. A blood albumin result cannot determine whether albumin is leaking through the kidneys. Urine albumin-to-creatinine ratio, or uACR, remains an important companion test when evaluating possible kidney damage and chronic kidney disease risk.
eGFR Versus Creatinine
Creatinine is the measured concentration of a waste product in the blood. eGFR uses that creatinine result, together with age and sex, to estimate kidney filtration.
The same creatinine value may have a different meaning in two people of different ages or sexes. eGFR helps translate the raw creatinine concentration into a more clinically interpretable estimate.
However, creatinine and eGFR are not independent markers. A change in creatinine automatically changes eGFR in the opposite direction. Seeing both change does not represent two separate pieces of evidence.
Creatinine provides the measured data, while eGFR provides an interpretation of that measurement through a population-based equation.
Frequently Asked Questions
Does an eGFR from 60 to 89 mean I have kidney disease?
Not necessarily. This range is mildly lower than young-adult levels but does not meet the criteria for chronic kidney disease without persistent albuminuria, structural kidney abnormalities, or another marker of kidney damage. Age and trends over time also matter.
Does one eGFR below 60 diagnose chronic kidney disease?
No. Chronic kidney disease generally requires reduced filtration or another abnormality lasting at least three months. However, a substantially low or rapidly declining result may indicate an acute problem and should be evaluated promptly rather than simply waiting three months.
Can dehydration lower eGFR?
Yes. Dehydration can reduce kidney blood flow or concentrate creatinine, producing a temporarily lower eGFR. Repeat testing after normal hydration and recovery from illness may help determine whether the change persists.
Can muscle mass affect eGFR?
Yes. High muscle mass can make creatinine-based eGFR appear lower, while very low muscle mass can make it appear higher. Cystatin C or a combined creatinine-cystatin C estimate may be useful when muscle mass makes creatinine difficult to interpret.
Is a higher eGFR always better?
No. eGFR should not be maximized. Once filtration is within an expected range, a higher estimate does not necessarily mean healthier kidneys. Very high values are also less precise and may require context.
Why is eGFR not appropriate during pregnancy?
Pregnancy changes kidney blood flow, filtration, and creatinine concentration in ways that standard equations do not model accurately. Kidney function during pregnancy should therefore be assessed using pregnancy-specific clinical interpretation rather than a standard eGFR result.
Conclusion
eGFR provides a practical estimate of how efficiently the kidneys filter the blood. By translating creatinine through age- and sex-specific calculations, it offers more context than creatinine alone.
Rythm displays an eGFR from 90 to 140 as optimal, values from 60 to 89 as normal, values below 60 as out of range, and values below 15 as critical. These classifications provide useful screening guidance, but eGFR remains an estimate rather than a direct measurement or standalone diagnosis.
When interpreted alongside creatinine, urine albumin, health history, physiological context, and longitudinal trends, eGFR provides valuable insight into kidney filtration and how it changes over time.
References
Inker LA, Eneanya ND, Coresh J, Tighiouart H, Wang D, Sang Y, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. The New England Journal of Medicine. 2021;385(19):1737–1749. doi:10.1056/NEJMoa2102953.
Kidney Disease: Improving Global Outcomes CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International. 2024;105(Suppl 4S):S117–S314.
Delgado C, Baweja M, Crews DC, Eneanya ND, Gadegbeku CA, Inker LA, 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. American Journal of Kidney Diseases. 2022;79(2):268–288.e1. doi:10.1053/j.ajkd.2021.08.003.
National Institute of Diabetes and Digestive and Kidney Diseases. eGFR Equations for Adults. Last reviewed May 2025.
National Institute of Diabetes and Digestive and Kidney Diseases. Factors Affecting eGFR Accuracy. Last reviewed May 2025.
Wiles K, Chappell L, Clark K, Elman L, Hall M, Lightstone L, et al. Clinical practice guideline on pregnancy and renal disease. BMC Nephrology. 2019;20:401. doi:10.1186/s12882-019-1560-2.