An eGFR, or estimated glomerular filtration rate, is a number that tells you how well your kidneys are filtering your blood. It appears on routine blood work and is measured in milliliters per minute. A normal eGFR is 90 or above, and lower numbers mean your kidneys are working less efficiently.
Your kidneys contain roughly one million tiny filtering units called glomeruli. Every minute, these filters process a surprising volume of blood, removing waste products and excess fluid that leave your body as urine. The eGFR estimates how much blood passes through those filters each minute, giving a snapshot of overall kidney function.
How eGFR Is Calculated
You can’t measure filtration rate directly inside a living kidney. Instead, a lab measures a waste product called creatinine in your blood. Creatinine is produced naturally by your muscles and filtered out by your kidneys at a fairly constant rate. When your kidneys slow down, creatinine builds up, so higher creatinine levels signal lower kidney function.
A formula then converts that creatinine level into an eGFR score, factoring in your age and sex. The current standard formula, adopted in 2021 after a joint recommendation by the National Kidney Foundation and the American Society of Nephrology, is race-neutral. Earlier versions included a race variable, but that practice has been eliminated from clinical guidelines.
Because creatinine comes from muscle, certain situations can throw off the estimate. People with unusually high muscle mass may get a lower eGFR than their actual kidney function warrants, while older adults or those with chronic illness and low muscle mass may get a falsely reassuring number. In those cases, doctors sometimes order a second marker called cystatin C, a protein produced by all cells rather than just muscle, to get a more accurate picture.
What the Numbers Mean
Kidney function is divided into five stages based on eGFR. These stages help categorize how much filtering capacity remains:
- Stage 1 (eGFR 90 or above): Normal filtering, though other signs of kidney damage like protein in the urine may be present.
- Stage 2 (eGFR 60 to 89): Mild loss of function. Many people at this stage have no symptoms.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss of function.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss of function.
- Stage 4 (eGFR 15 to 29): Severe loss of function. Preparation for possible dialysis or transplant typically begins here.
- Stage 5 (eGFR below 15): Kidney failure. The kidneys can no longer sustain the body’s needs without dialysis or a transplant.
A single low reading doesn’t automatically mean chronic kidney disease. The formal definition requires an eGFR below 60, or other markers of kidney damage like protein in the urine, documented for more than three months. A temporarily low result can happen during dehydration, acute illness, or after taking certain medications.
How eGFR Changes With Age
Kidney function naturally declines as you get older. Starting around age 30 to 40, the average person loses roughly one point of eGFR per year. That means a healthy 70-year-old will typically have a lower eGFR than a healthy 30-year-old, and that difference is expected. This is one reason the eGFR formula includes age as a variable. An eGFR of 75 in a 25-year-old raises more concern than the same number in a 75-year-old.
What the Blood Test Involves
An eGFR comes from a standard blood draw, usually part of a basic or comprehensive metabolic panel. The lab measures your serum creatinine, then plugs it into the formula. Results are typically available within a day or two.
Preparation varies by provider, but you may be asked to fast for several hours beforehand or avoid eating meat for a day or two before the test. Meat, especially red meat, contains creatine that your body converts to creatinine, which can temporarily inflate your levels and make your eGFR look worse than it is. Some medications can also affect the result, so your provider may ask you to skip certain ones before the draw.
Estimated vs. Measured GFR
The “e” in eGFR stands for estimated, and that distinction matters. A measured GFR involves injecting a tracer substance into the bloodstream and tracking how quickly the kidneys clear it. This is far more precise but also time-consuming and impractical for routine screening. It’s reserved for situations where precision is critical, such as evaluating a potential kidney donor or when creatinine-based estimates are unreliable.
For the vast majority of people, the estimated version is accurate enough to guide clinical decisions. If your eGFR lands in a borderline range, or if your doctor suspects the creatinine-based estimate doesn’t match your clinical picture, they may order a cystatin C-based eGFR or, less commonly, a measured GFR to clarify.
Why Your eGFR Matters
Kidney disease is often called a silent condition because stages 1 through 3 rarely produce noticeable symptoms. Most people discover a low eGFR through routine lab work, not because they felt something was wrong. By the time symptoms like swelling, fatigue, or changes in urination appear, significant function has already been lost.
Tracking your eGFR over time is more useful than any single result. A stable eGFR of 65 over several years tells a very different story than one that dropped from 80 to 65 in six months. The trend reveals whether kidney function is holding steady, declining slowly as expected with age, or dropping at a pace that needs intervention. If your lab results include an eGFR, it’s worth keeping a record so you can compare numbers across visits.