What is a glomerular filtration rate (GFR) test?
A glomerular filtration rate (GFR) is a blood test that checks how well your kidneys are working. Your kidneys have tiny filters called glomeruli. These filters help remove waste and excess fluid from the blood. A GFR test estimates how much blood passes through these filters each minute.
A GFR can be measured directly, but it is a complicated test, requiring specialized providers. So GFR is most often estimated using a test called an estimated GFR or eGFR. To get an estimate, your provider will use a method known as a GFR calculator. A GFR calculator is a type of mathematical formula that estimates the rate of filtration using some or all of the following information about you:
- The results of a blood test that measures creatinine, a waste product filtered by the kidneys
The eGFR is a simple test that can provide very accurate results.
Other names: estimated GFR, eGFR, calculated glomerular filtration rate, cGFR
What is it used for?
A GFR test is used to help diagnose kidney disease at an early stage, when it is most treatable. GFR may also be used to monitor people with chronic kidney disease (CKD) or other conditions that cause kidney damage. These include diabetes and high blood pressure.
Why do I need a GFR test?
Early stage kidney disease doesn’t usually cause symptoms. But you may need a GFR test if you are at higher risk of getting kidney disease. Risk factors include:
Later stage kidney disease does cause symptoms. So you may need a GFR test if you have any of the following symptoms:
What happens during a GFR test?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
Will I need to do anything to prepare for the test?
You may need to fast (not eat or drink) or avoid certain foods for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.
Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
What do the results mean?
Your GFR results may show one of the following:
- Normal—you probably don’t have kidney disease
- Below normal—you may have kidney disease
- Far below normal—you may have kidney failure
If you have questions about your results, talk to your health care provider.
Is there anything else I need to know about a GFR test?
Although damage to kidneys is usually permanent, you can take steps to prevent further damage. Steps may include: