What Is Glomerular Filtration Rate?
The glomerular filtration rate (GFR) is a measure of how well your kidneys are removing waste from your blood, which can be determined using an estimated GFR (eGFR) test.
The test gets its name from glomeruli -- the parts of your kidney that filter out waste.
Testing a person's GFR directly requires many blood draws over a longer period, so most health care providers use an eGFR, which involves a single blood test and a calculation. An eGFR measures how much blood these filters clean every minute.
Glomerular filtration rate vs. creatinine clearance
A creatinine clearance test is one that looks at how hard your kidneys are working based on how much creatinine, a waste product, is in your blood and urine. This test requires a 24-hour collection of pee and having your blood drawn. It's one way to estimate your GFR.
Instead of a creatinine clearance test, doctors usually use an eGFR blood test and a one-time urine collection.
If your doctor thinks your eGFR blood test could be incorrect.
Normal Range for Glomerular Filtration Rate
| Age (range) | eGFR (average) mL/min/1.73 m2 |
| 20-29 years | 116 |
| 30-39 years | 107 |
| 40-49 years | 99 |
| 50-59 years | 93 |
| 60-69 years | 85 |
| 70+ years | 75 |
An average eGFR varies by age, but in younger adults who are considered healthy, a range of 80-120 mL/min/1.73 m2 is generally normal. GFR decreases as you get older, but a reduced GFR can cause similar issues in older adults as in those who are younger. A decrease in GFR could also be a sign of kidney disease or kidney failure. A GFR below 60 mL/min/1.73 m2 may indicate a higher risk for CKD complications as well as cardiovascular disease.
The normal GFR range also depends on your sex and body size.
Stages of GFR
The level of kidney damage that is shown by the results of an eGFR test can help your doctor determine the stage of kidney failure. There are five stages of kidney failure:
Stage I. GFR 90 or higher (up to 100). Your kidneys still work normally, but you may have mild damage.
Stage II. GFR 89 to 60. Your kidney function loss is mild, with your kidneys still working well.
Stage IIIa. GFR 59 to 45. The loss of kidney function may be mild to moderate.
Stage IIIb. GFR 44 to 30. Your kidney function loss is moderate to severe.
Stage IV. GFR 29 to 15. You have a severe loss of kidney function.
Stage V. GFR lower than 15. Your kidneys are close to or at complete failure.
Who Needs a GFR Test?
Your doctor may do an eGFR test as part of a regular physical exam, or they might recommend it if you have signs of kidney problems or risk factors for kidney disease. Those can include:
- Problems peeing, including pain
- Having to go more often or making less pee
- Blood, foam, or a brownish color in your pee
- Swelling or puffiness around your eyes, belly, wrists, or ankles
- Pain in the middle of your back, near your kidneys
Your doctor probably will also want to check your eGFR if you have a condition that affects your kidneys, such as certain autoimmune disorders, diabetes, high blood pressure, heart disease, frequent urinary tract infections, liver disease, cancer, or a history of smoking. You may also get an eGFR test if other people in your family have diabetes or kidney disease.
How the eGFR Test Works
Before your test, be sure to tell your doctor about any drugs, vitamins, or dietary supplements you take. They may tell you not to take certain medicines or eat certain foods beforehand.
For the test, a doctor or nurse will take a sample of your blood. Then they’ll send it to a lab to measure the creatinine level in it. To figure out your results, they’ll use a mathematical formula that’s based on the amount they find, along with your age and sex. Some formulas also use race as a factor, but some don't, as research shows that including this factor in testing caused some Black people to be diagnosed and treated for kidney failure at later stages. Another blood test for a molecule called cystatin C can also be used to determine eGFR, and the combination of using both creatinine and cystatin C tends to have the most accurate eGFR result.
In general, an eGFR below 60 mL/min/1.73 m² can be a sign of kidney disease. But your doctor will be able to give you more specific information about your result, taking all your health information into account.
How Is GFR Calculated?
A GFR blood test relies on measuring creatinine levels in your blood, which can be affected by muscle mass, obesity, age, poor nutrition, and certain long-term conditions. Cystatin C can also be used, as per above. It's hard to directly measure GFR as it is a difficult and long process, so doctors use an estimate based on creatinine and/or cystatin C. Estimating GFR includes:
Checking creatinine levels (a waste product created by the body from muscle metabolism and is removed from the body by the kidneys) and/or cystatin C level (a protein made by cells in your body; an increase in the level means your kidneys aren't working well). These levels are combined with age and sex to determine eGFR through a mathematical formula.
One equation used by health care providers to confirm your eGFR is the CKD-EPI Creatinine Equation:
eGFRcr = 142 x min(Scr/κ, 1)α x max(Scr/κ, 1)-1.200 x 0.9938Age x 1.012 (if female)
where:
Scr = standardized serum creatinine in milligrams per deciliter
κ = 0.7 (females) or 0.9 (males)
α = -0.241 (female) or -0.302 (male)
min(Scr/κ, 1) is the minimum of Scr/κ or 1.0
max(Scr/κ, 1) is the maximum of Scr/κ or 1.0
Age (years)
What Can Affect the GFR Test?
Your result might not be as accurate if you:
- Are under 18
- Are elderly
- Are pregnant
- Have a kidney condition or other serious illnesses
- Have more muscle than average, like a bodybuilder
- Have a disease that lowers your muscle mass
- Follow a vegetarian diet
- Are obese
If you’re in one of these groups, talk with your doctor about how your results might be affected.
What Foods Increase GFR?
Your diet can play a role in improving some health conditions, such as diabetes. Decreasing your salt intake can help with managing kidney disease. Here are some measures you can consider including in your diet to keep your kidneys healthy:
- Avoid adding salt to food while cooking, eating, or using flavored salt.
- Skip processed meats such as hot dogs and lunch meat, chicken nuggets, or canned soups.
- Keep your servings to no more than 300 milligrams of sodium.
- Choose lower salt options in general.
- Steer clear of meats that are stored in a solution, including frozen varieties.
Takeaways
GFR is a measure of how well your kidneys are filtering your blood. It's important to monitor GFR because changes in its range can be an indicator of kidney disease and can help prevent additional damage. Talk to your health care provider if you have signs of kidney damage, including trouble peeing, (including pain, blood, foam, or a brownish color in your pee), swelling or puffiness around your eyes, belly, wrists, or ankles, or pain in the middle of your back, near your kidneys. If your doctor notices a change in your kidney function from a blood and/or urine test, they can come up with a treatment plan for you that may include medications or changes in your diet.
Glomerular Filtration Rate FAQs
Will drinking water increase my GFR? Drinking water is important because it helps blood to flow to your kidneys; however, you do not need to overdo it as that will not help. You just need to avoid dehydration. As your GFR decreases naturally with age, focusing on healthy lifestyle habits, such as maintaining healthy blood pressure, blood sugar levels, and a healthy weight, can improve the general health of your kidneys.
Is it good to have a high glomerular filtration rate? Yes, in general, the higher the GFR, the better. Sometimes in the early stages of diabetic kidney disease, high GFR can be a bad thing, however. A GFR of 90 or higher is considered normal if you're healthy, unless you have other symptoms such as protein in your urine, which could still be a sign of kidney disease.
Where does glomerular filtration take place? The process of glomerular filtration happens in the glomerulus of the nephrons of your kidneys. Each nephron consists of a glomerulus (which filters the blood) and a tubule (which returns nutrients to your blood and gets rid of waste).