Estimated glomerular filtration rate (eGFR) is a number that tells you how well your kidneys are filtering your blood. A normal eGFR is 90 or above, and healthy kidneys filter about 120 milliliters of blood plasma per minute, which adds up to roughly 180 liters per day. The number appears on routine blood work and is the single most important measure doctors use to assess kidney function.
What Your Kidneys Actually Filter
Each kidney contains about a million tiny filtering units called glomeruli. These structures act like a sophisticated sieve: blood flows in, and the glomeruli separate waste products and excess fluid from the things your body needs to keep. The filters work by both size and electrical charge. Small molecules pass through easily, while larger proteins like albumin are held back. This is why protein showing up in urine is an early warning sign of kidney damage: the filters are letting through something they normally block.
GFR measures the volume of plasma that passes through these filters per minute. It’s the most direct reflection of how many filtering units are working and how efficiently they’re doing their job. When kidney tissue is damaged or lost, fewer glomeruli are available to filter blood, and eGFR drops.
How eGFR Is Calculated
Measuring true GFR directly is complex and impractical for routine care. Instead, labs estimate it using a blood test for creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys clear creatinine efficiently, so when it builds up in the blood, it signals that filtration has slowed.
The current standard formula is the 2021 CKD-EPI equation, which uses three variables: your serum creatinine level, age, and sex. A key update in 2021 was the removal of race as a variable. The National Kidney Foundation and American Society of Nephrology jointly recommended this change, and labs across the country adopted the race-free equation.
For greater accuracy, labs can also measure a second blood marker called cystatin C. Unlike creatinine, cystatin C is produced by all cells in the body rather than just muscle, so it’s less influenced by body composition. Using both creatinine and cystatin C together gives the most accurate estimate. Guidelines recommend the combined approach as the preferred method when precision matters, such as when making decisions about medications that depend on kidney function.
What the Numbers Mean: CKD Stages
Your eGFR result maps to a stage of chronic kidney disease (CKD). These stages help doctors gauge severity and decide on monitoring or treatment:
- Stage 1 (eGFR 90 or above): Normal filtering capacity, though kidney damage may still be present if there’s protein in the urine or structural abnormalities on imaging.
- Stage 2 (eGFR 60 to 89): Mild loss of function. Many people in this range have no symptoms and may never progress further.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss. This is often where closer monitoring begins.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss. Complications like anemia or bone changes become more likely.
- Stage 4 (eGFR 15 to 29): Severe loss. Planning for possible dialysis or transplant typically starts 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 eGFR reading doesn’t automatically mean you have CKD. The diagnosis requires the abnormality to persist for at least three months. A temporary drop can happen from dehydration, certain medications, or an acute illness, and kidney function may bounce back once the cause resolves.
Why Your eGFR Might Be Misleading
Because creatinine is a byproduct of muscle metabolism, anything that affects your muscle mass can skew the result. A bodybuilder with high muscle mass produces more creatinine, which can make their eGFR look lower (worse) than their kidneys actually warrant. Conversely, someone who is elderly, malnourished, or has significant muscle loss may show a falsely reassuring eGFR because their creatinine is naturally low.
Diet plays a role too. Eating a large amount of cooked meat before a blood draw can temporarily raise creatinine. Liver disease and significant fluid overload also lower creatinine levels, inflating eGFR. Certain medications can raise creatinine by blocking its secretion in the kidneys without actually changing filtration, creating the appearance of worsening function when nothing has changed.
This is exactly why cystatin C testing exists. For people at the extremes of muscle mass, older adults with sarcopenia (age-related muscle loss), or anyone whose creatinine-based result seems inconsistent with the clinical picture, a cystatin C measurement provides a useful second opinion.
eGFR and Age
Kidney function naturally declines with age. A 25-year-old and a 75-year-old with identical creatinine levels will get different eGFR results because the equation adjusts for age. Even healthy adults lose some filtering capacity over the decades. This means an eGFR of 65 in someone who is 80 carries a different clinical significance than the same number in someone who is 35.
This age dimension also affects referral decisions. Most international guidelines recommend referral to a kidney specialist when eGFR drops below 30, regardless of age. But some guidelines take a more nuanced approach, using higher eGFR thresholds for younger patients (who have more to lose from early kidney decline) and reserving specialist referral for lower numbers in older adults, where a modest reduction may simply reflect normal aging rather than progressive disease.
What Happens After a Low Result
If your eGFR comes back low, the first step is usually a repeat test in a few weeks or months to confirm the result. A single low reading can reflect temporary factors like dehydration or a recent illness. Your doctor will also check for protein in your urine, since the combination of low eGFR and elevated urine protein together paints a much clearer picture of kidney health than either test alone.
For confirmed CKD at stage 3 or beyond, monitoring typically includes periodic blood and urine tests to track the rate of decline. The speed at which eGFR drops over time matters as much as the absolute number. Someone whose eGFR has been stable at 50 for years is in a very different situation than someone whose eGFR has fallen from 70 to 50 in twelve months. A rapid decline, generally defined as a loss of more than 5 points per year, raises more concern and may prompt earlier referral to a nephrologist.
At stage 4 (eGFR 15 to 29), most guidelines recommend specialist involvement to prepare for potential kidney replacement therapy. At this stage, discussions about dialysis options or transplant evaluation typically begin, even though the kidneys may still function adequately for months or years.