GFR, or glomerular filtration rate, measures how well your kidneys filter waste from your blood. It’s expressed in milliliters per minute, and a normal value for a young, healthy adult falls roughly between 90 and 120 mL/min. In most cases, GFR isn’t measured directly. Instead, it’s estimated from a simple blood test that checks how much of a waste product called creatinine is in your bloodstream. Your lab plugs that number into a formula, and the result is your estimated GFR, or eGFR.
How eGFR Is Calculated From a Blood Test
The most common way to determine GFR is through a standard blood draw. The lab measures your serum creatinine, a byproduct your muscles produce at a fairly steady rate. Healthy kidneys filter creatinine out efficiently, so when levels rise in the blood, it signals that filtration has slowed down.
That creatinine value is then run through a mathematical equation that adjusts for your age and sex to produce an eGFR number. The current standard equation is the CKD-EPI formula, recommended by the international KDIGO guidelines since 2013. It replaced an older formula called the MDRD equation, which tended to be less accurate at higher filtration rates. The CKD-EPI equation performs better across the full range of kidney function, making it more reliable for people whose kidneys are still working relatively well.
In 2021, a joint task force from the National Kidney Foundation and the American Society of Nephrology recommended that all U.S. laboratories adopt an updated version of the CKD-EPI equation that no longer includes a race variable. This race-free equation was developed using a diverse population and is now the standard approach nationwide.
Cystatin C: A Second Blood Marker
Creatinine has a significant limitation: its levels depend heavily on muscle mass. A bodybuilder and a frail older adult could have the same kidney function but very different creatinine levels, leading to misleading eGFR results. This is where cystatin C comes in. It’s a small protein produced by nearly all cells in the body at a relatively constant rate, making it less influenced by muscle mass, age, or sex than creatinine.
An eGFR calculated from both creatinine and cystatin C together is generally accepted as more accurate than either marker alone. The KDIGO guidelines recommend cystatin C testing as a confirmatory step when creatinine-based results seem unreliable or when a clinical decision hinges on the exact GFR number. Your doctor might order a cystatin C test if you have unusually high or low muscle mass, eat a very high-protein or vegetarian diet, or take medications that interfere with creatinine measurement.
Cystatin C isn’t perfect either. Steroid use, thyroid disorders, obesity, and inflammation can all push cystatin C levels up independent of kidney function.
Factors That Can Throw Off Your Results
Because eGFR is an estimate, several things can make it less accurate. Understanding these helps you interpret your results and, in some cases, prepare properly before the blood draw.
- Muscle mass: People with significantly more or less muscle than average (bodybuilders, people with amputations, those with paraplegia, or frail older adults) may get eGFR values that don’t reflect true kidney function.
- Diet: A very high-protein meal can temporarily raise creatinine, while a long-term vegetarian or low-meat diet may keep creatinine unusually low, making kidneys appear healthier than they are.
- Medications: Certain drugs interfere with the lab assay itself, not just kidney function. The older Jaffe method of measuring creatinine is particularly sensitive to interference from medications, high blood sugar, and ketone bodies in people with uncontrolled diabetes.
- Body size extremes: eGFR is typically adjusted to a standard body surface area. For people with obesity or very small body size, this adjustment may not be accurate enough without additional correction.
If any of these apply to you, a cystatin C-based eGFR or a direct GFR measurement may give a clearer picture.
The 24-Hour Urine Collection
Another way to assess kidney filtration is creatinine clearance, which uses both a blood sample and a full day’s worth of urine. You’ll be given a large container and asked to collect every drop of urine over a 24-hour period, keeping the container refrigerated or cool throughout. The collection typically starts by emptying your bladder in the morning (discarding that first sample) and then saving everything from that point until the same time the next morning.
The lab compares the amount of creatinine in your urine to the amount in your blood, producing a clearance rate that approximates GFR. This method is less commonly used now because it depends entirely on a complete, accurate collection. Missing even one bathroom trip during the 24 hours will skew the results. Some medications, supplements, and foods can also interfere, so your provider may ask you to avoid certain items during the collection period.
Measured GFR: The Most Accurate Method
For situations where precision truly matters, GFR can be measured directly rather than estimated. This involves injecting a harmless tracer substance into a vein and then drawing blood samples over several hours to see how quickly the kidneys clear it. The most commonly used tracer today is iohexol, a low-cost contrast agent with a strong safety profile. Iothalamate is another option.
Measured GFR (mGFR) is the gold standard, but it’s time-consuming and impractical for routine screening. It’s reserved for specific clinical situations: evaluating whether someone with advanced kidney disease is ready for a transplant waiting list or dialysis access, adjusting the dose of medications that are cleared by the kidneys and have a narrow margin of safety, or monitoring the precise response to treatments aimed at slowing kidney disease progression. A GFR below 20 mL/min, for instance, may prompt consideration of transplant listing, and an accurate measurement at that level can change the clinical plan.
What the Numbers Mean: CKD Stages
Once you have an eGFR value, it maps to a stage of chronic kidney disease. These stages help guide monitoring and treatment decisions.
- Stage 1 (eGFR 90 or above): Normal filtration rate, but other signs of kidney damage may be present, such as protein in the urine.
- Stage 2 (eGFR 60 to 89): Mildly reduced function with evidence of kidney damage.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss of function.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss of function.
- Stage 4 (eGFR 15 to 29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure.
It’s worth noting that a single eGFR reading doesn’t automatically mean you have kidney disease. The diagnosis typically requires the abnormality to persist for at least three months, sometimes alongside other markers like protein in the urine or structural changes visible on imaging.
How Age Affects Normal GFR
GFR naturally declines with age, and this is not the same thing as kidney disease. In healthy adults, kidney filtration drops by roughly 0.4 mL/min per year. A study from the Nijmegen Biomedical Study measured eGFR in healthy people across different age groups and found clear, predictable declines.
For healthy women, the average eGFR was about 100 in the 18-to-24 age group, dropping to around 83 by the late 40s, 75 by the mid-60s, and 62 after age 85. For healthy men, the pattern was similar but slightly lower overall: about 91 in the youngest group, 74 by the late 40s, 66 by the late 60s, and 59 after 85.
This means an eGFR of 68 in a healthy 65-year-old man is entirely normal, even though it would technically fall in the Stage 2 range. Context matters. A single number without considering age, trends over time, and other kidney markers doesn’t tell the full story. If your eGFR has been stable at a level appropriate for your age and you have no protein in your urine, that’s a very different situation from an eGFR that’s dropped 15 points in two years.