An eGFR blood test estimates how well your kidneys are filtering waste from your blood. The “e” stands for estimated, and GFR stands for glomerular filtration rate, which is the volume of blood your kidneys clean each minute. A normal eGFR is 90 or above, though the number naturally declines with age. It’s one of the most common tests used to screen for and monitor kidney disease.
How the Test Works
Rather than measuring kidney filtration directly (which requires a complicated procedure), your provider estimates it from a simple blood draw. The lab measures creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys filter creatinine out efficiently, so when it builds up in your blood, it signals that your kidneys aren’t keeping up.
The lab plugs your creatinine level into a formula along with your age and sex to calculate your eGFR. The current standard formula, adopted in 2021, no longer includes a race variable. The National Kidney Foundation recommended this change so that all patients receive the same eGFR result from the same creatinine level, regardless of race.
No special preparation is typically needed. You generally don’t have to fast before the blood draw, and results usually come back within a day or two as part of a routine metabolic panel.
What the Numbers Mean
Your eGFR result is reported in milliliters per minute, representing how much blood your kidneys filter each minute. Higher is better. The stages of chronic kidney disease (CKD) are defined by these ranges, according to the National Kidney Foundation:
- Stage 1 (eGFR 90 or above): Normal kidney function, though other signs of damage (like protein in urine) may be present
- Stage 2 (eGFR 60 to 89): Mildly reduced function
- Stage 3a (eGFR 45 to 59): Mild to moderate reduction
- Stage 3b (eGFR 30 to 44): Moderate to severe reduction
- Stage 4 (eGFR 15 to 29): Severely reduced function
- Stage 5 (eGFR below 15): Kidney failure, where dialysis or transplant becomes necessary
A single low reading doesn’t automatically mean you have chronic kidney disease. CKD is typically diagnosed when your eGFR stays below 60 for three months or more, or when other markers of kidney damage persist alongside a reduced number. Your provider will likely repeat the test before drawing conclusions.
How Age Affects Your Results
Kidney function declines naturally over your lifetime, even without disease. According to Cleveland Clinic data, the average eGFR for someone in their twenties is around 116, while the average for someone over 70 drops to about 75. That means a 72-year-old with an eGFR of 78 is in a very different situation than a 30-year-old with the same number. Your provider interprets your result in the context of your age, not just against a single cutoff.
When the Test Can Be Misleading
Because eGFR is calculated from creatinine, anything that changes your creatinine level for reasons unrelated to kidney health can throw off the estimate. Muscle mass is the biggest culprit. Creatinine is a byproduct of muscle metabolism, so people with significantly more muscle produce more of it. Research on this effect found that eGFR was falsely reduced by about 6 points for every 10 kilograms (roughly 22 pounds) of lean mass. In highly muscular people, nearly half of “abnormal” eGFR results were actually false positives for kidney disease.
On the other end, people with very low muscle mass, such as those with severe weight loss from chronic illness, can have a falsely elevated eGFR that masks real kidney problems. In that group, the test missed about a third of people who actually had stage 3 kidney disease. Certain medications can also raise creatinine levels without affecting true kidney function.
Body fat, interestingly, doesn’t affect the accuracy of the test. The errors are driven specifically by lean muscle tissue.
Cystatin C: A More Accurate Alternative
When your provider suspects the creatinine-based eGFR might not reflect your true kidney function, they can order a test based on a different blood marker called cystatin C. Unlike creatinine, cystatin C isn’t influenced by muscle mass, making it more reliable for bodybuilders, very thin or elderly patients, and people at the extremes of body composition.
Cystatin C-based eGFR also tends to be more accurate in the high-normal range of kidney function, which matters for catching early decline. This is especially relevant for people with diabetes, where creatinine-based eGFR has well-documented limitations. Recent clinical guidelines have pushed for broader use of cystatin C, either on its own or combined with creatinine in a single equation for the most precise estimate available.
Not every lab runs cystatin C routinely, so it’s typically ordered as a follow-up when standard results seem inconsistent with the clinical picture.
Why Your Provider Orders This Test
An eGFR shows up in several common scenarios. It’s part of routine bloodwork for people with diabetes, high blood pressure, heart disease, or a family history of kidney problems, since these conditions are the leading causes of kidney damage. It’s also used to monitor people already diagnosed with CKD, track the effects of medications that can stress the kidneys, and evaluate kidney function before certain imaging procedures that use contrast dye.
If your eGFR comes back low, expect your provider to order a urine test checking for protein (a separate marker of kidney damage) and to repeat the blood test in a few months. A single low number in isolation often turns out to be temporary, caused by dehydration, a recent high-protein meal, or a new medication. The trend over time matters far more than any individual result.