GFR on a blood test stands for glomerular filtration rate, a number that tells you how well your kidneys are filtering waste from your blood. A normal GFR is about 90 or above, and anything below 60 for three months or longer points to chronic kidney disease. It’s one of the most common lab values you’ll see on routine bloodwork, and understanding what your number means can tell you a lot about your kidney health.
What GFR Actually Measures
Your kidneys contain millions of tiny filters called glomeruli. Every minute, these filters process your blood, pulling out waste products and excess water and sending them out through urine. GFR measures how many milliliters of blood your kidneys filter per minute. A higher number means your kidneys are working efficiently. A lower number means filtration has slowed down, which can allow waste to build up in your bloodstream.
Measuring GFR directly is complicated and rarely done outside of research settings. Instead, your lab calculates an estimated GFR (eGFR) from a standard blood draw. The test measures creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys clear creatinine quickly, so when creatinine levels rise in your blood, it signals that your kidneys aren’t filtering as well as they should. The lab plugs your creatinine level, age, and sex into a formula called the CKD-EPI equation to produce your eGFR number.
Until 2021, this formula included a race variable that adjusted results for Black patients. A joint task force from the National Kidney Foundation and the American Society of Nephrology recommended removing race from the equation, recognizing that race is a social construct rather than a biological one. All U.S. labs now use the updated race-free CKD-EPI 2021 equation.
What’s Considered Normal
A GFR of 90 or higher is considered normal kidney function. For healthy young adults up to age 40, the average measured GFR is about 107 mL/min. After 40, kidney filtration naturally declines at a rate of roughly 1 mL/min per year. So a 60-year-old with a GFR in the mid-80s could have perfectly healthy kidneys for their age.
Here’s what the age-related decline looks like in healthy people based on measured (not estimated) GFR data from potential kidney donors:
- Ages 20 to 40: average GFR around 107
- Ages 40 to 50: average GFR around 104
- Ages 50 to 60: average GFR around 90 to 97
- Ages 60 to 70: average GFR around 80 to 90
This matters because a GFR of 75 in a 65-year-old carries a different meaning than the same number in a 30-year-old. Context is everything when reading your result.
GFR Ranges and Kidney Disease Stages
Doctors classify chronic kidney disease into five stages based on your eGFR. These stages help guide how aggressively kidney health needs to be monitored and managed.
- Stage 1 (GFR 90 or above): Normal filtration, but other signs of kidney damage may be present, like protein in the urine.
- Stage 2 (GFR 60 to 89): Mildly reduced function. Often detected incidentally on routine bloodwork. Many people at this stage have no symptoms.
- Stage 3a (GFR 45 to 59): Mild to moderate loss of function. This is typically where closer monitoring begins.
- Stage 3b (GFR 30 to 44): Moderate to severe loss of function.
- Stage 4 (GFR 15 to 29): Severe loss of function. Planning for possible dialysis or transplant often starts here.
- Stage 5 (GFR below 15): Kidney failure. The kidneys can no longer sustain normal body function on their own.
A single low reading doesn’t automatically mean kidney disease. The diagnosis requires a GFR below 60 sustained over at least three months, or evidence of kidney damage like protein in the urine even with a normal GFR.
Symptoms at Different GFR Levels
One of the tricky things about kidney disease is that it’s often silent in the early stages. Most people with a GFR between 60 and 89 feel completely fine, which is why the condition is frequently caught through routine blood tests rather than symptoms.
As GFR drops below 30, symptoms become more likely. You might notice swelling in your feet and ankles from fluid retention, persistent fatigue, nausea, loss of appetite, muscle cramps, dry and itchy skin, or changes in how often you urinate. Shortness of breath can develop if fluid accumulates in the lungs. Mental sharpness can also decline, and blood pressure may become harder to control. These symptoms overlap with many other conditions, so they aren’t a reliable way to self-diagnose kidney problems.
What Can Throw Off Your Results
Because eGFR is calculated from creatinine, anything that changes your creatinine level can skew the result without reflecting actual kidney function. Muscle mass is a major factor. People with significantly more muscle produce more creatinine, which can make their eGFR appear lower than it truly is. The reverse is true for people with very low muscle mass, whose eGFR may look artificially high.
Diet and exercise also play a role. Eating a large amount of red meat or taking creatine supplements increases creatinine production. Intense exercise can temporarily spike creatinine levels, sometimes dramatically. One study found that creatinine-based eGFR dropped by 16% immediately after a half-marathon, and another showed a 30% drop after just 30 minutes of vigorous cycling. These changes are temporary, driven by creatinine release from working muscles, dehydration, and reduced blood flow to the kidneys during exertion.
If your eGFR comes back borderline or unexpectedly low, your doctor may order a repeat test or check a different blood marker called cystatin C. Unlike creatinine, cystatin C isn’t significantly affected by muscle mass, diet, or exercise, making it a useful confirmation test. Guidelines specifically recommend cystatin C testing for people whose creatinine-based eGFR falls between 45 and 60 but who don’t have other signs of kidney disease like protein in the urine.
Preparing for the Test
An eGFR blood draw is straightforward. No fasting is required for the creatinine measurement itself, though your doctor may ask you to fast if other tests are being run at the same time, like cholesterol or blood sugar. The blood draw takes a few minutes, and results are typically available within a day or two.
For the most accurate result, avoid intense exercise for 24 to 48 hours before the test. If you take creatine supplements or regularly eat very high-protein meals, mention that to your doctor, as it could affect interpretation. Staying normally hydrated (not over-hydrating or dehydrated) gives the cleanest baseline reading.