Glomerular filtration rate (GFR) is a measure of how well your kidneys are filtering your blood. It tells you how many milliliters of blood plasma your kidneys clean per minute, with a healthy range of roughly 80 to 120 mL/min in younger adults. When you see “eGFR” on a blood test report, the “e” stands for estimated, because the lab calculates your filtration rate from a simple blood draw rather than measuring it directly.
What GFR Actually Measures
Your kidneys contain roughly one million tiny filtering units called glomeruli. Each one is a knot of capillaries that lets water, salts, and small waste molecules pass through into a collection space while holding back larger proteins and blood cells. The glomerulus acts like a selective sieve: molecules below a certain size slip through, while the filter’s negative electrical charge repels most proteins back into the bloodstream. GFR captures the total output of all these filters working together, making it the single best indicator of overall kidney function.
When GFR drops, waste products that would normally leave through your urine start accumulating in your blood. Fluid balance, blood pressure regulation, and electrolyte levels all depend on adequate filtration, which is why doctors track this number closely.
How the Lab Calculates Your eGFR
True GFR can only be measured by injecting a special tracer substance and tracking how fast the kidneys clear it, a process too cumbersome for routine care. Instead, labs estimate GFR using a waste product already in your blood: creatinine. Creatinine is a byproduct of normal muscle activity, and healthy kidneys filter it out at a predictable rate. A standard blood draw measures your creatinine level, and a formula converts that number into an eGFR.
The recommended formula in the United States is the 2021 CKD-EPI creatinine equation, which factors in your age and sex but no longer includes a race variable. Earlier versions adjusted the result based on race, a practice that was widely criticized and has since been dropped by the National Kidney Foundation.
There is also a second marker called cystatin C, a small protein filtered by the kidneys. Your doctor may order a cystatin C test if the creatinine-based estimate seems unreliable. This is most common in people whose muscle mass is unusually high or low, such as bodybuilders, the very elderly, or people with paralysis, since creatinine levels are heavily influenced by how much muscle you carry. When both markers are combined in a single equation, the estimate is more precise than either one alone.
What the Numbers Mean
eGFR is reported in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m²). Higher numbers mean better filtration. The stages of chronic kidney disease are defined by these cutoffs:
- Stage 1 (eGFR 90 or above): Normal kidney function. A result in this range alone does not mean kidney disease unless other signs of damage, like protein in the urine, are present.
- Stage 2 (eGFR 60 to 89): Mildly decreased function. Like Stage 1, this does not count as chronic kidney disease by itself without additional markers of damage.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss of function. This is often the stage where further testing, such as cystatin C, is ordered to confirm the diagnosis.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss of function.
- Stage 4 (eGFR 15 to 29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure. At this level the kidneys can no longer sustain life without dialysis or a transplant.
How eGFR Changes With Age
Kidney filtration naturally declines as you get older, so a result that would be concerning in a 25-year-old may be perfectly expected at 75. Average eGFR values by decade illustrate the trend:
- Ages 20 to 29: approximately 116 mL/min
- Ages 30 to 39: approximately 107 mL/min
- Ages 40 to 49: approximately 99 mL/min
- Ages 50 to 59: approximately 93 mL/min
- Ages 60 to 69: approximately 85 mL/min
- Ages 70 and older: approximately 75 mL/min
An eGFR of 75 in a 72-year-old is typical, while the same number in a 30-year-old warrants a closer look. Your doctor interprets the result in the context of your age, health history, and whether your number is trending downward over time.
Symptoms of Low GFR
Early kidney function loss rarely causes noticeable symptoms, which is why routine blood work catches problems that you would otherwise miss. By the time you feel something, kidney disease is often advanced. Symptoms of significantly reduced filtration include persistent fatigue, nausea, loss of appetite, swelling in the feet or ankles, trouble sleeping, and difficulty concentrating. High blood pressure that resists treatment is another common sign. If fluid builds up in the lungs, shortness of breath can develop.
These symptoms occur because the kidneys can no longer remove enough waste, fluid, and excess minerals from the blood. Potassium, phosphate, and acids accumulate, disrupting processes throughout the body.
What Can Throw Off Your Results
Because the standard eGFR calculation relies on creatinine, anything that changes your creatinine level for reasons unrelated to your kidneys can skew the number. Muscle mass is the biggest factor. People with very high muscle mass may get a falsely low eGFR, while those with very low muscle mass (from malnutrition, aging, or chronic illness) may get a falsely high one. Certain medications can also block creatinine secretion in the kidneys, making levels rise without any actual change in filtration. A high-protein meal, particularly one heavy in cooked meat, can temporarily raise creatinine as well.
If your doctor suspects your result is inaccurate for any of these reasons, a cystatin C test or a combined creatinine-cystatin C calculation provides a clearer picture. Cystatin C is not influenced by muscle mass the way creatinine is, making it especially useful in people at the extremes of body composition.
Preparing for the Test
Your provider will give you specific instructions, but you may be asked to fast or avoid certain foods for several hours before the blood draw. Make sure to mention all medications and supplements you take, since some can affect creatinine levels. Do not stop any medication on your own before the test.
eGFR in Children and Young Adults
The adult CKD-EPI formula is not designed for children. For patients ages 1 to 25, labs use a different calculator called CKiD U25, which accounts for the fact that children’s kidneys and body size are still developing. For young adults between 18 and 25, doctors sometimes run both the pediatric and adult calculators side by side to get the most accurate picture. Normal GFR in a healthy child is typically higher than in an older adult, so pediatric results require their own reference ranges.