Your GFR, or glomerular filtration rate, is a measure of how well your kidneys are filtering your blood. Specifically, it estimates how many milliliters of blood your kidneys’ tiny filters (called glomeruli) clean each minute. A normal GFR for most healthy adults falls between 90 and 120, though this number naturally decreases with age. If you’ve seen a GFR number on recent lab work, understanding where it falls can tell you a lot about your kidney health.
How Your Kidneys Produce a GFR Number
Your kidneys contain roughly a million microscopic filtering units called glomeruli. Blood flows into these filters under pressure, and they separate waste products and excess fluid from substances your body needs to keep, like proteins and blood cells. The filtered waste becomes urine. GFR captures the total volume of fluid your glomeruli filter per minute, making it the single best indicator of overall kidney function.
You won’t see “GFR” measured directly on most lab tests. Instead, you’ll see “eGFR,” the estimated version. Labs calculate your eGFR from a simple blood draw that measures creatinine, a waste product your muscles produce at a fairly steady rate. The current standard formula, adopted in 2021, uses your creatinine level along with your age and sex to generate the estimate. No race variable is included in the updated equation, a change recommended jointly by the National Kidney Foundation and the American Society of Nephrology to improve consistency across populations.
A second blood marker called cystatin C can also be used. Combining creatinine and cystatin C produces a more accurate estimate than creatinine alone, and guidelines recommend this combination when a more precise assessment is needed. Your doctor might order the combined test if your creatinine-based eGFR seems inconsistent with your overall health picture.
What a Normal GFR Looks Like by Age
A GFR of 90 or above is generally considered normal, but “normal” shifts as you get older. Your kidneys lose some filtering capacity with each decade, even without any disease. According to Cleveland Clinic data, here’s what average eGFR looks like across age groups:
- Ages 20 to 29: average eGFR of 116
- Ages 30 to 39: average eGFR of 107
- Ages 40 to 49: average eGFR of 99
- Ages 50 to 59: average eGFR of 93
- Ages 60 to 69: average eGFR of 85
- Ages 70 and over: average eGFR of 75
This means a 72-year-old with an eGFR of 78 is in a completely different situation than a 35-year-old with the same number. Context matters, and a single reading below 90 doesn’t automatically signal kidney disease.
GFR Ranges and Kidney Disease Stages
Chronic kidney disease (CKD) is classified into five stages based on your eGFR. These stages help doctors gauge how much filtering capacity your kidneys have lost and what kind of monitoring or treatment you might need.
- Stage 1 (eGFR 90 or above): Kidney function is normal, but there may be other signs of damage, like protein in the urine.
- Stage 2 (eGFR 60 to 89): Mild loss of kidney function. Many people at this stage have no symptoms at all.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss of function. This is often when monitoring becomes more frequent.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss of function. Complications like anemia or bone weakness may start to develop.
- Stage 4 (eGFR 15 to 29): Severe loss of function. Planning for possible dialysis or transplant typically begins here.
- Stage 5 (eGFR below 15): Kidney failure. The kidneys can no longer sustain life without dialysis or a transplant.
One important detail: stages 1 and 2 require evidence of actual kidney damage (protein in urine, structural abnormalities on imaging, or other markers) to qualify as CKD. A mildly reduced eGFR on its own doesn’t necessarily mean you have kidney disease, especially if you’re older.
What Low GFR Feels Like
In the early stages, kidney disease is almost always silent. Most people with stage 1, 2, or even early stage 3 CKD feel perfectly fine, which is why routine blood work catches it more often than symptoms do.
As kidney function drops further, waste products build up in the blood and the body struggles to maintain its chemical balance. Advanced CKD can cause fatigue and weakness, nausea, loss of appetite, trouble sleeping, and difficulty concentrating. Fluid retention can lead to swollen legs or shortness of breath if fluid backs up into the lungs. Blood pressure often becomes harder to control. Over time, CKD can affect nearly every organ system, contributing to heart disease, weakened bones, anemia, and changes in immune function.
These symptoms tend to develop gradually, and many of them overlap with other common conditions, which makes the eGFR blood test so valuable. It can flag declining kidney function years before you’d notice anything wrong.
What Can Temporarily Shift Your eGFR
Because eGFR is an estimate, several factors can push the number up or down without reflecting a true change in kidney health. Creatinine, the blood marker used in the most common calculation, is influenced by muscle mass. A very muscular person may have higher creatinine levels (and therefore a lower eGFR) that don’t indicate kidney problems. The reverse is true for someone with very low muscle mass, whose eGFR might appear artificially high.
Diet also plays a role. Eating a large amount of protein, especially red meat, can temporarily raise creatinine levels. Dehydration reduces blood flow to the kidneys and can lower your eGFR on a given day. Intense or prolonged exercise has a similar effect by redirecting blood flow away from the kidneys toward working muscles, though a typical moderate workout is unlikely to cause a meaningful shift.
Certain medications and even some supplements can interfere with creatinine measurements. If your eGFR comes back unexpectedly low, your doctor may simply repeat the test or order the cystatin C-based version to confirm the result before drawing conclusions.
Estimated GFR vs. Measured GFR
For the vast majority of people, the estimated version (eGFR) from a standard blood draw is accurate enough for clinical decisions. But in certain situations, the estimate can be unreliable. People with extreme body compositions, those experiencing rapidly changing kidney function (like during a hospitalization for acute kidney injury), or people with thyroid conditions that affect cystatin C levels may need a more precise approach.
In those cases, doctors can perform a measured GFR test, which involves injecting a special tracer substance into the bloodstream and tracking how quickly the kidneys clear it. This is more time-consuming and not widely available, so it’s reserved for situations where the standard estimate isn’t trustworthy enough to guide important treatment decisions. True GFR can never be known with absolute certainty, but the measured version gets closer than any equation can.