GFR (glomerular filtration rate) is a direct measurement of how well your kidneys filter blood, while eGFR (estimated glomerular filtration rate) is a calculated approximation of that same value using a simple blood test. In everyday medical care, you’ll almost always see eGFR on your lab results because true GFR measurement is complex, expensive, and reserved for situations where precision really matters. Both numbers use the same unit (mL/min/1.73 m²), and both tell you the same thing: how efficiently your kidneys are working.
How True GFR Is Measured
A true, or “measured,” GFR (sometimes written as mGFR) has long been considered the gold standard for assessing kidney function. The process involves injecting a special substance into your bloodstream, then tracking how quickly your kidneys filter it out. The substances used are chosen specifically because the kidneys remove them only through filtration, not through any other route. Common options include iohexol (a contrast agent), iothalamate, and inulin.
Because these substances are cleared exclusively by kidney filtration, the rate at which they disappear from your blood gives a precise picture of how your kidneys are performing. The downside is practical: the test requires an injection, multiple blood draws or urine collections over several hours, and specialized lab analysis. It’s not something you’d do at a routine checkup.
How eGFR Is Calculated
eGFR skips the injection entirely. Instead, it uses a blood marker your body produces naturally, most commonly creatinine, a waste product generated by your muscles. Your doctor orders a standard blood draw, the lab measures your creatinine level, and a mathematical formula converts that number into an estimated filtration rate.
The formula also factors in your age and sex, since these influence how much creatinine your body produces independent of kidney function. The most widely used formula today is the CKD-EPI 2021 equation, recommended by the National Kidney Foundation and the American Society of Nephrology. Notably, this updated equation removed a race-based coefficient that older formulas (like the original CKD-EPI and MDRD equations) had included, after a task force concluded that the race variable was not scientifically justified and could lead to disparities in care.
Some labs also calculate eGFR using a different blood marker called cystatin C, a small protein produced by nearly all cells in the body. Cystatin C is less influenced by muscle mass than creatinine, which can make it more accurate in certain people. A combined equation using both creatinine and cystatin C is also available and tends to be the most precise of the estimation options.
Why eGFR Can Be Inaccurate
Because eGFR is built on assumptions about what’s “typical” for someone of your age and sex, it can miss the mark when you don’t fit the population the equation was designed around. Creatinine-based eGFR is particularly sensitive to factors that change how much creatinine your body produces or how it’s metabolized. These include:
- Muscle mass: Very muscular people produce more creatinine, which can make eGFR appear lower (worse) than their actual kidney function. Conversely, people with low muscle mass, including many older adults, may get eGFR results that look better than reality.
- Diet: High protein intake, especially from meat, raises creatinine levels and can temporarily lower your eGFR reading.
- Body size: The standard eGFR is adjusted to an average body surface area of 1.73 m². If your body size is significantly larger or smaller, the estimate may not reflect your true filtration rate.
- Medications: Certain drugs interfere with how creatinine is handled by the kidneys, shifting the eGFR result without any real change in kidney function.
In a study of older adults, creatinine-based eGFR overestimated true kidney function by a median of about 11 mL/min/1.73 m² in the majority of participants, a clinically meaningful gap. Cystatin C-based eGFR was closer to measured GFR in that group, with a median bias of only about 3 mL/min/1.73 m². However, cystatin C has its own blind spots: it can be affected by thyroid dysfunction, inflammation, obesity, and steroid use, though these influences are less well understood than those affecting creatinine.
When Measured GFR Is Worth the Effort
For the vast majority of people, eGFR is accurate enough to guide medical decisions. Clinical guidelines recommend it as the standard approach and suggest reserving measured GFR for situations where greater accuracy is needed. Those situations typically include:
- Kidney donor evaluation: Before someone donates a kidney, doctors need to know their exact filtration rate to ensure the remaining kidney can handle the workload.
- Borderline results: When eGFR places someone right at the threshold between kidney disease stages, a measured GFR can clarify whether treatment needs to change.
- Unusual body composition: Amputees, bodybuilders, people with severe malnutrition, or those with rapidly changing weight may get misleading eGFR results.
- Drug dosing for narrow-margin medications: Some drugs are cleared almost entirely by the kidneys, and getting the dose wrong can cause toxicity. Measured GFR reduces that risk.
How Both Numbers Are Used to Stage Kidney Disease
Whether obtained through measurement or estimation, GFR is the basis for staging chronic kidney disease. The stages use the same cutoffs regardless of how the number was obtained:
- Stage 1 (GFR 90 or above): Normal or high filtration, though kidney damage may still be present based on other signs like protein in the urine.
- Stage 2 (GFR 60 to 89): Mildly decreased function.
- Stage 3a (GFR 45 to 59): Mild to moderate decrease.
- Stage 3b (GFR 30 to 44): Moderate to severe decrease.
- Stage 4 (GFR 15 to 29): Severely decreased function.
- Stage 5 (GFR below 15): Kidney failure.
A normal GFR for a healthy young adult is around 90 to 120 mL/min/1.73 m². GFR naturally declines with age, so a value of 70 in a 75-year-old may not carry the same significance as the same number in a 30-year-old. Staging always considers GFR alongside other markers, particularly whether there’s protein or blood in the urine.
What Your Lab Results Actually Show
When you look at a standard metabolic panel or kidney function test, the number labeled “eGFR” is almost certainly a creatinine-based estimate. Your lab will not perform a measured GFR unless it’s specifically ordered. If you see two eGFR values, one is likely creatinine-based and the other cystatin C-based. The combined equation using both markers tends to be the most accurate single estimate available and is increasingly recommended when cystatin C testing is accessible.
If your eGFR comes back lower than expected and you’re otherwise healthy, it’s worth considering whether any of the accuracy factors listed above apply to you. A single eGFR reading is a snapshot, not a definitive diagnosis. Kidney disease is typically confirmed by repeat testing over at least three months, along with urine tests to check for signs of kidney damage beyond filtration rate alone.