What kidney function tests measure and how to do the math yourself
Kidney function is measured by how well your kidneys filter waste from your blood. The standard way to express this is eGFR — estimated glomerular filtration rate — which tells you how many milliliters of blood your kidneys can clean per minute. You can calculate your own eGFR using a straightforward formula, your age, sex, and a single blood test result called creatinine.
The most common formula used in the United States is the CKD-EPI equation (Chronic Kidney Disease Epidemiology Collaboration). It takes your serum creatinine level — measured in milligrams per deciliter (mg/dL) — and adjusts it for your age, sex, and race. Your doctor's lab report will include your creatinine number. From there, the math takes less than a minute.
Key Takeaways
- eGFR is calculated from your serum creatinine level, age, sex, and race using the CKD-EPI formula, which is the standard in U.S. medical practice.
- You need only one number from your blood work — serum creatinine in mg/dL — to plug into the formula yourself.
- The CKD-EPI equation produces different results for Black and non-Black patients because of differences in muscle mass and creatinine production, though this approach is evolving.
- An eGFR above 60 is generally considered normal kidney function; below 60 suggests some loss of function that your doctor should monitor.
- Online eGFR calculators do the arithmetic for you, but understanding the formula helps you read your own lab results.
The CKD-EPI formula and what each number means
The CKD-EPI equation is:
eGFR = 142 × (Scr/0.7)^a × (0.9839)^age
Where Scr is your serum creatinine level, a is an exponent that depends on your sex (−0.241 for women, −0.302 for men), and age is your age in years. If your creatinine is higher than 0.7 mg/dL, you use the number as-is. If it is lower, you use 0.7 as the baseline instead.
The formula also includes a race-based multiplier: multiply the result by 1.0 if you are Black, or 1.0 if you are not Black (meaning no multiplier is applied). This distinction exists because research showed that Black patients tend to have higher creatinine levels for the same level of kidney function, but this approach is being reconsidered by medical organizations. Some labs now report eGFR without the race multiplier.
The result is a number in mL/min/1.73m². The "1.73m²" part is a standardized body surface area used to make results comparable across people of different sizes.
Step-by-step calculation with a real example
Suppose you are a 55-year-old woman with a serum creatinine of 0.9 mg/dL. Here is how to calculate your eGFR:
- Divide your creatinine by 0.7: 0.9 ÷ 0.7 = 1.286
- Raise that result to the power of −0.241 (the female exponent): 1.286^(−0.241) = 0.966
- Raise 0.9839 to the power of your age: 0.9839^55 = 0.352
- Multiply all three parts together: 142 × 0.966 × 0.352 = 48.2
Your eGFR would be approximately 48 mL/min/1.73m². This falls into Stage 3b chronic kidney disease, meaning your kidneys are working at about half their normal capacity. Your doctor would want to monitor this and may order follow-up tests.
If you are not comfortable doing exponents by hand, use an online eGFR calculator — the National Kidney Foundation, Mayo Clinic, and many hospital systems offer free ones. You enter your age, sex, creatinine, and race, and the calculator does the arithmetic.
What your eGFR number means for kidney health
eGFR is divided into five stages that tell you how much kidney function remains:
| eGFR Range | Stage | What It Means |
|---|---|---|
| 90 or higher | Stage 1 | Normal kidney function, but other signs of kidney disease may be present |
| 60–89 | Stage 2 | Mild loss of kidney function; usually no symptoms |
| 45–59 | Stage 3a | Moderate loss of kidney function; doctor should monitor regularly |
| 30–44 | Stage 3b | Moderate to significant loss of kidney function; increased risk of complications |
| 15–29 | Stage 4 | Severe loss of kidney function; preparation for dialysis or transplant may begin |
| Below 15 | Stage 5 | Kidney failure; dialysis or transplant needed |
A single eGFR result is a snapshot. Your doctor looks at trends over time — if your eGFR drops by more than 30 percent in a year, that is a sign of rapid decline and warrants investigation. Stable eGFR, even if it is in Stage 3, is less concerning than a sharp drop.
Why creatinine is used and what affects it
Creatinine is a waste product made by muscle tissue. Your kidneys filter it out, so the amount in your blood reflects how well your kidneys are working. A higher creatinine level usually means lower kidney function.
However, creatinine is not a perfect marker. It depends on how much muscle you have — a muscular person produces more creatinine than a thin person with the same kidney function. This is why the formula adjusts for sex (men have more muscle on average) and why some doctors order a more precise test called cystatin C for people with unusual muscle mass, such as athletes or people with severe obesity.
Certain medications and supplements can also raise creatinine temporarily without meaning your kidneys are failing. NSAIDs like ibuprofen, ACE inhibitors, and some antibiotics can do this. If your creatinine jumps unexpectedly, your doctor may repeat the test after stopping the medication to see if it was the cause.
When to ask your doctor about your eGFR
If your eGFR is below 60, your doctor should discuss what it means and whether you need follow-up testing or lifestyle changes. You may need to adjust how much protein you eat, monitor your blood pressure more closely, or avoid certain medications.
If your eGFR drops by 25 percent or more from your previous result, tell your doctor even if the number is still in the normal range. A rapid decline can signal a new problem that needs investigation.
If you have diabetes, high blood pressure, or a family history of kidney disease, your doctor may check your eGFR regularly even if it is normal. These conditions put you at higher risk, and catching early changes matters.
Frequently Asked Questions
Can I calculate my eGFR if I do not know my race?
Yes. The race-based multiplier in the CKD-EPI formula is optional and increasingly questioned by medical organizations. You can calculate eGFR using 1.0 as the multiplier for everyone, or ask your lab whether they report eGFR with or without the race adjustment. Either way, the number tells you how your kidneys are filtering.
Does a normal eGFR mean my kidneys are completely healthy?
Not necessarily. eGFR measures only one aspect of kidney function — how fast they filter. You can have a normal eGFR and still have kidney disease if you have protein in your urine or other signs of damage. Your doctor looks at eGFR alongside urine tests and blood pressure to get the full picture.
How often should I have my eGFR checked?
If your eGFR is normal and you have no risk factors, once a year during a routine physical is typical. If you have diabetes, high blood pressure, or a previous eGFR below 60, your doctor may check it more often — sometimes every three to six months — to watch for changes.
What is the difference between eGFR and actual GFR?
eGFR is an estimate based on a formula. Actual GFR requires a 24-hour urine collection or an inulin clearance test, which is more accurate but less practical. For most people, eGFR is accurate enough to guide treatment decisions. Your doctor may order the more precise test if the eGFR result does not match your symptoms or other test results.
Can my eGFR improve if I lose weight or build muscle?
Losing weight may lower your creatinine slightly because you have less muscle tissue, which could raise your eGFR on paper. However, the real goal is to slow or stop kidney damage, which happens through blood pressure control, blood sugar management if you have diabetes, and avoiding nephrotoxic drugs. Talk to your doctor about what changes will actually protect your kidneys.