Glomerular filtration rate, commonly called GFR, is a measure of how well your kidneys filter waste from your blood. It tells you how much blood passes through your kidneys’ tiny filters each minute. A normal GFR in adults is above 90, though it naturally decreases with age. When doctors check your kidney function, this is the number they rely on most.
What GFR Actually Measures
Your kidneys contain roughly one million tiny filtering units called glomeruli. Blood flows into these filters under pressure, and waste products, excess fluid, and electrolytes get pushed out into the urine while useful proteins and blood cells stay behind. GFR captures the total volume of fluid filtered through all of these structures per minute. A higher number means your kidneys are filtering efficiently. A lower number means they’re struggling.
In practice, you’ll almost never see a “measured” GFR on your lab results. Directly measuring it requires injecting a tracer substance and tracking how quickly your kidneys clear it, which is time-consuming and expensive. Instead, doctors use a blood test to calculate an estimated GFR, written as eGFR. This estimate is based on the level of a waste product in your blood (usually creatinine), combined with your age and sex.
Normal Ranges by Age
GFR is reported in milliliters per minute, and normal values decline steadily as you get older, even in people with perfectly healthy kidneys. According to the National Kidney Foundation, average eGFR values by age look like this:
- Ages 20 to 29: 116
- Ages 30 to 39: 107
- Ages 40 to 49: 99
- Ages 50 to 59: 93
- Ages 60 to 69: 85
- Ages 70 and older: 75
So a 72-year-old with a GFR of 78 is in perfectly normal territory, while that same number in a 25-year-old would be a red flag. Context matters when interpreting the result.
What Low GFR Numbers Mean
Chronic kidney disease is staged entirely by GFR. The lower the number, the more advanced the kidney damage:
- 90 or above: Normal kidney function (though other signs of damage like protein in the urine can still indicate early disease)
- 60 to 89: Mildly reduced function
- 45 to 59: Mild to moderate loss
- 30 to 44: Moderate to severe loss
- 15 to 29: Severely reduced function
- Below 15: Kidney failure, where dialysis or a transplant typically becomes necessary
One of the trickiest things about kidney disease is that most people feel completely fine until their kidneys are close to failing. There are usually no symptoms in the early and middle stages, which is why routine blood work catches problems that you wouldn’t notice on your own.
How the Test Works
The standard eGFR test requires only a routine blood draw. The lab measures your creatinine level, a waste product generated by normal muscle activity, and plugs it into a formula along with your age and sex. The most widely used formula today is the 2021 CKD-EPI equation, which was updated to remove a race-based adjustment that had been part of earlier versions. The newer equation uses creatinine, age, and sex alone.
Some labs also use a second blood marker called cystatin C, a small protein produced at a steady rate by virtually all cells in your body. Unlike creatinine, cystatin C isn’t affected by muscle mass, making it useful in situations where creatinine alone might be misleading. Research has shown that cystatin C is a better predictor of kidney disease progression, particularly at GFR levels of 60 or below and in people whose creatinine doesn’t reliably reflect their true kidney function.
What Can Throw Off Your Results
Because eGFR relies heavily on creatinine, anything that changes your creatinine level independent of kidney function can skew the number. Muscle mass is the biggest factor. Bodybuilders and people with large frames tend to produce more creatinine, which can make their kidney function appear worse than it is. On the flip side, people with very low muscle mass (frail older adults, those with muscle-wasting conditions, amputees, or people eating a vegetarian or very low-meat diet) may have artificially low creatinine, making their eGFR look better than reality.
Pregnancy can also affect results because blood volume and kidney function shift dramatically. Hospitalized patients, people with acute kidney injury, and anyone whose kidney function is changing rapidly will get unreliable eGFR readings since the formula assumes a stable state.
Several common medications can raise creatinine levels without actually harming the kidneys. Certain heartburn drugs, antibiotics like trimethoprim, corticosteroids, and high-dose aspirin all interfere with how creatinine is handled in the body. If your eGFR drops after starting a new medication, your doctor may recheck it or order a cystatin C-based estimate to get a clearer picture. For the cystatin C test, steroid use, thyroid problems, obesity, and inflammation can reduce its accuracy.
What to Make of Your Number
A single eGFR result is a snapshot, not a diagnosis. Kidney function fluctuates day to day based on hydration, recent meals, and other factors. Doctors typically look for a trend over multiple tests before drawing conclusions. Two or more readings below 60, taken at least three months apart, generally point toward chronic kidney disease.
If your eGFR is between 60 and 89 with no protein in your urine and no other signs of kidney damage, it often reflects normal aging rather than disease. Below 60, further evaluation becomes important regardless of age. And if your result sits below 15, that signals the kidneys can no longer keep up with the body’s needs, and treatment options like dialysis or transplant enter the conversation.
Your eGFR is almost always reported automatically with routine blood work, so you may already have several results in your medical record without realizing it. Checking the trend over time gives you a much more useful picture than any single number.