GFR, or glomerular filtration rate, is calculated using a blood test that measures creatinine (a waste product from muscle breakdown) along with your age and sex. You don’t calculate it yourself. A lab runs the blood sample, plugs your creatinine level into a standardized equation, and reports back an estimated GFR, or eGFR, measured in milliliters per minute. A normal result is 90 or above.
What the Equation Actually Uses
The standard equation used in the United States is the CKD-EPI 2021 creatinine equation. It needs three inputs: your serum creatinine level from a blood draw, your age, and your sex. The formula applies different constants depending on whether you’re male or female, then adjusts downward as age increases, reflecting the natural decline in kidney filtration over time.
Until 2021, GFR equations included a race adjustment that gave different results for Black patients. A joint task force from the National Kidney Foundation and the American Society of Nephrology unanimously recommended removing race from the calculation. The current CKD-EPI 2021 equation does not use race, was developed with a diverse population, and is now the standard in all U.S. laboratories.
How Labs Turn a Blood Draw Into a Number
Your kidneys filter creatinine out of your blood at a fairly constant rate. When your kidneys work well, creatinine levels stay low. When filtration slows, creatinine builds up. The equation works backward from that principle: it takes your creatinine concentration and estimates how quickly your kidneys must be filtering to produce that level, adjusting for age and sex because those factors influence how much creatinine your body generates in the first place.
The full formula looks like this: eGFR = 142 × min(Scr/K, 1)^α × max(Scr/K, 1)^−1.200 × 0.9938^Age × 1.012 (if female). The constants K and α differ by sex. For females, K is 0.7 and α is −0.241. For males, K is 0.9 and α is −0.302. In practice, you never need to do this math. Online calculators from the National Kidney Foundation and NIDDK let you enter creatinine, age, and sex to get an instant result.
What Your eGFR Number Means
Kidney disease is staged entirely by eGFR ranges:
- Stage 1: eGFR of 90 or higher. Kidneys filter normally, though other signs of damage (like protein in the urine) may be present.
- Stage 2: eGFR between 60 and 89. A mild decrease in filtration.
- Stage 3a: eGFR between 45 and 59. Mild to moderate loss.
- Stage 3b: eGFR between 30 and 44. Moderate to severe loss.
- Stage 4: eGFR between 15 and 29. Severe loss of kidney function.
- Stage 5: eGFR below 15. Kidney failure, where dialysis or transplant typically becomes necessary.
A single low reading doesn’t necessarily mean chronic kidney disease. Doctors typically repeat the test over three months before assigning a stage, because creatinine can fluctuate from day to day based on hydration, diet, and other temporary factors.
When Creatinine-Based eGFR Can Be Wrong
The equation assumes your body produces creatinine at a predictable rate for someone of your age and sex. When that assumption breaks down, the estimate becomes unreliable. People with unusually high or low muscle mass are the most affected group. Bodybuilders, for instance, generate more creatinine than average and can appear to have worse kidney function than they actually do. On the other end, people with muscle-wasting conditions, amputations, paraplegia, or severe malnutrition produce less creatinine and can get falsely reassuring results.
Other situations that reduce accuracy include pregnancy, rapidly changing kidney function (such as during hospitalization for acute kidney injury), cirrhosis, nephrotic syndrome, and being over age 70. A vegetarian or low-meat diet can lower creatinine slightly, since dietary protein contributes to creatinine production. Certain medications and even elevated blood sugar in diabetic patients can interfere with the creatinine measurement itself in the lab.
The Cystatin C Alternative
When creatinine-based eGFR is likely to be inaccurate, doctors can order a cystatin C blood test instead. Cystatin C is a small protein produced by nearly all cells in the body at a steady rate. Unlike creatinine, its blood levels are not greatly influenced by muscle mass, age, sex, or diet. This makes it a more reliable marker for people who are very obese, malnourished, elderly, or have abnormal muscle mass for any reason.
Cystatin C also responds more quickly to changes in kidney function than creatinine does, making it more useful when filtration is shifting rapidly, such as in hospitalized patients. The CKD-EPI equation has a cystatin C version as well as a combined creatinine-cystatin C version, both of which can be calculated through the same online tools. Some labs will report both numbers side by side. If the two estimates differ significantly, the combined equation or the cystatin C result alone is generally considered more trustworthy.
One limitation: factors like steroid use, thyroid dysfunction, obesity-related inflammation, and certain inflammatory conditions can raise cystatin C levels independent of kidney function, so it’s not immune to interference either.
GFR in Children
The adult CKD-EPI equation doesn’t apply to children. Pediatric GFR is estimated using the Bedside Schwartz equation, which has been the standard since the 1970s and was updated with modern creatinine measurement methods. It uses a simpler formula: eGFR = k × height (in cm) / serum creatinine, where k is a constant of approximately 0.413 for children measured with current enzymatic creatinine assays. Height is included because it correlates with muscle mass in growing children better than age alone does.
Estimated vs. Measured GFR
Everything above describes estimated GFR, which is what routine blood work provides. There is also a directly measured GFR, which involves injecting a tracer substance into the bloodstream and tracking how quickly the kidneys clear it. This is far more accurate but also more time-consuming and expensive, so it’s reserved for specific clinical situations: evaluating living kidney donors, dosing certain toxic medications precisely, or resolving cases where eGFR results are unreliable or conflicting.
Preparing for the Blood Test
Your provider may ask you to fast or avoid certain foods for several hours before the blood draw, since a recent high-protein meal can temporarily raise creatinine. Strenuous exercise in the hours before the test can also elevate creatinine. Make sure your provider knows all the medications and supplements you’re taking, as some can affect either the creatinine level itself or the lab’s ability to measure it accurately. Don’t stop any medication without being told to do so.