GFR, or glomerular filtration rate, is measured in one of two ways: through a simple blood test that feeds into a mathematical formula (called estimated GFR or eGFR), or through a more involved clinical procedure that tracks how your kidneys clear a specific substance from your blood (called measured GFR or mGFR). The vast majority of people will only ever encounter the estimated version, which is the standard test ordered during routine bloodwork and kidney checkups.
What GFR Actually Tells You
GFR represents how much blood your kidneys filter per minute, expressed in milliliters per minute per 1.73 square meters of body surface area. A normal GFR is 90 or above. Think of it as a performance score for your kidneys: the higher the number, the better they’re working. When GFR drops, it means waste products are building up in your blood because your kidneys can’t keep pace.
Estimated GFR: The Standard Blood Test
Almost every GFR result you’ll see on a lab report is an estimate. Your doctor orders a blood draw, the lab measures a waste product in your blood, and a formula converts that measurement into your eGFR. The whole process requires nothing more than a standard blood sample.
The waste product most commonly measured is creatinine, a byproduct of normal muscle activity that your kidneys continuously filter out. If your kidneys are healthy, creatinine levels stay low. If they’re struggling, creatinine accumulates, and your eGFR drops. The current standard formula, called the 2021 CKD-EPI equation, uses your creatinine level along with your age and sex to generate the estimate. Notably, this updated equation removed the race-based adjustment that older versions included, making it a single formula applied equally to all patients.
A second blood marker called cystatin C can be used instead of, or alongside, creatinine. Cystatin C is a small protein produced at a steady rate by virtually every cell in your body. Unlike creatinine, it isn’t influenced by muscle mass. That makes it more reliable for people who are very muscular, very thin, elderly, or have conditions like liver cirrhosis that throw off creatinine readings. Research in the Clinical Kidney Journal has shown cystatin C to be more sensitive at detecting early kidney disease and more strongly linked to future cardiovascular risk. Guidelines from KDIGO and NICE recommend cystatin C as a confirmatory test when creatinine-based results may be unreliable.
When both markers are combined, the 2021 CKD-EPI equation uses age, sex, serum creatinine, and serum cystatin C together, producing the most accurate estimate available without a specialized procedure.
Preparing for the Blood Test
Your provider may ask you to fast or avoid certain foods for several hours before the draw. Some medications can also affect results, so mention everything you’re taking, including supplements. Don’t stop any medication on your own before the test.
Measured GFR: The Gold Standard
Measured GFR works on a different principle. Instead of looking at waste your body produces naturally, a clinician injects a known substance into your bloodstream and then tracks how quickly your kidneys remove it through a series of timed blood draws. The substance most commonly used today is iohexol, a contrast agent. Inulin, a plant-based fiber, was the original gold-standard marker but has been largely discontinued.
The procedure requires some preparation. You’ll maintain your regular diet and medications but need to avoid iodinated contrast (the dye used in CT scans) for seven days beforehand. The test itself is time-consuming: after the injection, blood samples are drawn at specific intervals over several hours. Those samples are then analyzed using a mathematical model to calculate your exact filtration rate.
Measured GFR is more accurate than eGFR at every stage of kidney disease, but it’s especially valuable in early stages (stages 1 and 2) where eGFR can miss subtle declines. It can also catch rapid drops in filtration that signal conditions like early diabetic kidney disease. The tradeoff is availability and cost. Most hospitals don’t routinely offer mGFR testing, and it takes significantly more time and resources than a standard blood draw. It’s typically reserved for situations where precision matters most: evaluating living kidney donors, guiding treatment decisions in complex cases, or resolving ambiguous eGFR results.
Where eGFR Falls Short
Estimated GFR is a formula, and formulas have blind spots. Several conditions can make eGFR less reliable:
- Unusual muscle mass. Bodybuilders and people with muscle-wasting conditions produce abnormal amounts of creatinine, skewing the estimate in opposite directions.
- Pregnancy. Changes in blood volume and kidney function during pregnancy affect creatinine levels.
- Age over 70. Muscle mass naturally declines with age, which can make eGFR appear higher than the true filtration rate.
- Liver disease. Cirrhosis alters creatinine production.
- Organ transplant recipients. Immunosuppressive medications and the transplanted organ itself can influence results.
- Nephrotic syndrome. Losing large amounts of protein in urine changes the markers eGFR relies on.
In these situations, switching to a cystatin C-based estimate or pursuing a measured GFR provides a clearer picture.
How GFR Results Map to Kidney Disease Stages
Once you have a GFR number, it falls into one of six categories used to classify chronic kidney disease:
- G1 (GFR 90 or above): Normal or high function. This does not count as CKD on its own unless there’s other evidence of kidney damage, such as protein in the urine.
- G2 (GFR 60 to 89): Mildly decreased. Like G1, this alone doesn’t qualify as CKD without additional signs of damage.
- 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.
A single eGFR reading doesn’t establish a diagnosis. Kidney disease is typically confirmed when a low GFR persists for three months or more, often alongside other markers like elevated protein in the urine. Temporary drops can happen from dehydration, medication changes, or acute illness without indicating chronic disease.
Which Test You’re Likely to Get
For routine screening and ongoing monitoring, you’ll get a creatinine-based eGFR through a standard blood panel. It’s inexpensive, fast, and available at any lab. If your results are borderline or your doctor suspects the creatinine estimate may be unreliable for your body type or health situation, the next step is usually a cystatin C test, which is still a simple blood draw. Measured GFR with iohexol or another tracer is a specialized procedure you’d only encounter if there’s a specific clinical reason that demands precise numbers. For the vast majority of people monitoring their kidney health, eGFR provides a practical and sufficiently accurate answer.