GFR, or glomerular filtration rate, is most commonly measured through a simple blood test that checks your creatinine level, which is then plugged into a formula to produce an estimated GFR (eGFR). A normal eGFR in adults is above 90 mL/min/1.73 m², though this number naturally declines with age. There are also more precise methods involving urine collection or specialized infusions, but the blood test is what most people encounter at a routine checkup.
What GFR Actually Tells You
Your kidneys filter blood through tiny clusters of blood vessels called glomeruli. GFR measures how much blood those filters clean per minute. Think of it as a performance score for your kidneys. A higher number means your kidneys are filtering efficiently; a lower number means they’re struggling.
GFR is reported in milliliters per minute, adjusted for body surface area. It’s the single most important number for diagnosing and staging chronic kidney disease.
The Standard Blood Test (eGFR)
The most common way to measure GFR is indirect. A healthcare provider draws blood, measures the level of creatinine (a waste product your muscles produce at a fairly steady rate), and enters that value into a mathematical formula along with your age and sex. The result is your estimated GFR.
The formula used in most labs today is the 2021 CKD-EPI creatinine equation, developed by the National Kidney Foundation and the American Society of Nephrology. A key update in 2021 removed the race coefficient that older versions included, which had been widely criticized. The current equation factors in serum creatinine, age, and sex to produce an eGFR score.
Your provider may ask you to fast or avoid certain foods for several hours before the blood draw, since a large meal (especially one heavy in meat) can temporarily raise creatinine levels and skew the result.
Why Creatinine Isn’t Always Reliable
Creatinine works well as a kidney marker for most people, but it has blind spots. Your creatinine level depends heavily on how much muscle you have. A bodybuilder and a frail older adult could have the same kidney function but very different creatinine readings. Aging naturally reduces both muscle mass and creatinine production, which can mask a real decline in kidney function since the blood level stays stable even as filtration drops.
Diet also matters. Meat is a major source of creatinine, and reducing meat intake can lower creatinine excretion by 10 to 30 percent. Protein supplements can push levels higher. Pregnancy raises GFR by about 50 percent in early stages compared to later in pregnancy, so timing matters for interpretation.
Several medications can raise creatinine levels without actually affecting kidney function. These include certain heart rhythm drugs, some HIV medications, and the common antibiotic combination trimethoprim-sulfamethoxazole. If you’re taking any of these, your eGFR may look worse than your kidneys truly are.
Cystatin C: A Second Blood Marker
When creatinine-based estimates aren’t trustworthy, providers can order a second blood marker called cystatin C. This protein is produced by nearly all cells in the body at a steady rate and filtered by the kidneys, making it less dependent on muscle mass, age, or sex than creatinine.
An eGFR calculated from both creatinine and cystatin C together is generally more accurate than either marker alone. Cystatin C is particularly useful for confirming a diagnosis when the creatinine result seems off, or when decisions about treatment hinge on knowing the exact GFR.
Cystatin C has its own limitations. Chronic inflammation, obesity, smoking, and thyroid problems can all falsely elevate it. It also costs more than a creatinine test, and many labs worldwide still don’t routinely offer it.
24-Hour Urine Collection
A more hands-on method involves collecting every drop of urine you produce over a full 24-hour period. You’ll receive a large plastic container and, depending on your sex, a urinal or collection hat that sits over the toilet. You save all urine for exactly 24 hours, then return the container to the lab. A blood sample is drawn at the same time to measure serum creatinine.
By comparing how much creatinine appears in your urine versus your blood, the lab calculates your creatinine clearance, which closely approximates GFR. The accuracy depends on collecting every sample. If you accidentally discard even one void, you need to let your provider know, because the result will underestimate your kidney function.
This method is less convenient than a simple blood draw, but it provides a direct measurement rather than an estimate. It’s often used when eGFR results are uncertain or when precise dosing of medications cleared by the kidneys is critical.
The Gold Standard: Inulin Clearance
The most accurate way to measure GFR involves infusing a substance called inulin directly into a vein. Inulin is an ideal marker because it dissolves in water, doesn’t bind to proteins, passes freely through the kidney’s filters, and is neither reabsorbed nor secreted by the kidney tubules. It measures what the kidney actually filters, with no biological shortcuts.
The test requires trained personnel and is typically done in the morning after an overnight fast. After an initial loading dose, a steady infusion maintains a stable blood concentration of 300 to 400 mg/L. Blood and urine samples are collected at carefully timed intervals. Patients are encouraged to drink water throughout the procedure to increase urine flow, which improves accuracy. In some cases, a bladder catheter is needed to ensure complete urine collection.
Inulin clearance is considered the reference method against which all other GFR tests are compared. It’s rarely used in everyday clinical practice because it’s time-consuming, expensive, and requires IV access. You’d typically only encounter it in a research setting or a specialized kidney center.
How GFR Changes With Age
Even in healthy people, GFR declines over time. Average eGFR values by age look roughly like this:
- Ages 20 to 29: 116 mL/min
- Ages 30 to 39: 107 mL/min
- Ages 40 to 49: 99 mL/min
- Ages 50 to 59: 93 mL/min
- Ages 60 to 69: 85 mL/min
- Ages 70 and older: 75 mL/min
This means a 72-year-old with an eGFR of 78 is right in the normal range for their age, even though the number falls below the standard threshold of 90. Context matters when interpreting your result.
GFR and Kidney Disease Stages
GFR is the basis for staging chronic kidney disease. The international classification system breaks it into six categories:
- G1 (GFR 90 or above): Normal or high function
- G2 (GFR 60 to 89): Mildly decreased
- G3a (GFR 45 to 59): Mild to moderate decrease
- G3b (GFR 30 to 44): Moderate to severe decrease
- G4 (GFR 15 to 29): Severely decreased
- G5 (GFR below 15): Kidney failure
An important detail: a GFR in the G1 or G2 range alone does not qualify as chronic kidney disease. There must also be evidence of kidney damage, such as protein in the urine or structural abnormalities on imaging. A mildly decreased GFR in an older adult, without other signs of damage, is often a normal part of aging rather than a disease diagnosis.
How GFR Is Measured in Children
Standard adult formulas don’t work for children because kids have different body proportions and their kidneys are still developing. Pediatric eGFR relies on a height-based equation. The most widely used version, sometimes called the Bedside Schwartz formula, multiplies a constant (41.3) by the child’s height in meters, then divides by their serum creatinine level. It was developed using data primarily from children ages 8 to 15 with mild to moderate kidney disease and is designed for quick clinical estimates.
For newborns up to four weeks old, a separate neonatal equation uses a smaller constant (0.31) with height in centimeters. Newer equations for patients under 25 incorporate age and sex-specific adjustments for greater precision as children grow into adulthood.