Glomerular Filtration Rate (GFR) Test

What is a glomerular filtration rate (GFR) test?

A glomerular filtration rate (GFR) test is a blood test that checks how well your kidneys are working. Your kidneys are two organs on either side of your spine near your waist. They have tiny filters called glomeruli. These filters remove waste and extra water from your blood and gets rid of them through urine (pee).

If your kidneys have been damaged by kidney disease, they can't filter your blood as fast as they should. A GFR test checks for kidney disease by measuring how much blood your kidneys filter each minute.

GFR can be measured directly, but it is a complicated test to do. So health care providers usually estimate GFR based on the amount of certain waste substances in your blood. An estimated GFR is called an eGFR.

To figure out your eGFR, your provider usually uses the results of a blood test that measures your creatinine level. Creatinine is a waste product that comes from normal wear and tear on your muscles. If your kidneys aren't working well, creatinine can build up in your blood.

People make different amounts of creatinine, depending on their size, diet, and activity levels. So, to calculate your eGFR, your provider will use your creatinine levels and other information about you, such as your:

• Age
• Weight
• Height
• Sex

This information is put into a mathematical formula, called a GFR calculator, to find your eGFR.

eGFR may also be calculated using the level of cystatin C in your blood. This is a protein that many cells in your body make. Cystatin C levels are not affected by muscle size, age, or diet, so some researchers think cystatin C provides a more accurate estimate of GFR than creatinine. In certain cases, creatinine and cystatin levels are both used to calculate eGFR in adults.

Other names: estimated GFR, eGFR, calculated glomerular filtration rate, cGFR

What is it used for?

A GFR test is used to:

• Screen for kidney disease in people without symptoms
• Help diagnose kidney disease in a people who have symptoms
• Help find out how serious kidney disease is
• Monitor people who:
  • Have chronic kidney disease (CKD) to see if treatment is helping
  • Take medicines that could harm their kidneys
• Check kidney health before people start certain treatments that could affect their kidneys

Why do I need a GFR test?

Early-stage kidney disease doesn't usually cause symptoms, so you may have an eGFR test to check your kidney health as part of a routine exam.

You may also need this test if you have a high risk of getting kidney disease. Your risk may be higher if you:

• Have diabetes
• Have high blood pressure
• Have a family health history of kidney disease, diabetes, or high blood pressure
• Have heart disease
• Are over 50 years old
• Smoke
• Have obesity

If you have a condition that increases you risk of kidney disease, ask your provider how often you should get tested.

Later stage kidney disease does cause symptoms. So, you may need an eGFR test if you have:

• Swelling in your legs, feet, ankles, or hands or face
• Urinating (peeing) more often or less often than usual
• Dry skin and/or itching
• Fatigue
• Muscle cramps
• Nausea and vomiting
• Loss of appetite

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?

Your provider will let you know how to prepare for your test. You may need to fast (not eat or drink) or avoid certain foods for several hours before the test. Certain medicines can affect your results. So be sure to tell your provider everything you're taking. But don't stop taking any medicine unless your provider tells you to stop.

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 eGFR results may be reported as a number that shows how much blood your kidneys filter per minute. Your test results may also give your creatinine level and/or your cystatin C level.

Ask your provider to explain what your eGFR says about your kidney health. An eGFR isn't a perfect test and may not always reflect kidney damage. It's also possible to have an abnormal result even if you don't have kidney damage.

In general:

• A normal eGFR means that you probably don't have kidney disease.
• An eGFR that's below normal or low may mean that you may have kidney disease.
• A very low eGFR means that you may have kidney failure.

If you're diagnosed with kidney disease, your provider will explain what you can do to protect your kidneys from more damage and help prevent kidney failure.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a GFR test?

Your provider may order other tests that check your kidney health. These tests may be ordered with an eGFR or to find the cause of an abnormal eGFR result. They include:

• A protein in urine test. Protein in urine is a sign of kidney damage.
• A microalbumin creatinine ratio test. This test checks for very small amounts of a protein called albumin in a sample of your urine. Albumin in urine may be one of the first signs of kidney disease.
• A BUN (blood urea nitrogen). BUN is a waste product your kidneys remove from blood. The test checks BUN levels in a sample of your blood.