What Is a Glomerular Filtration Rate (GFR)?

The glomerular filtration rate, or GFR, is a measure of how well your kidneys are cleaning your blood -- taking out waste and extra water. An estimated GFR test (eGFR) can tell your doctor how much of a certain waste product called creatinine is in your blood.

The test gets its name from the parts of your kidney that filter waste out -- they’re called glomeruli.

Who Needs This Test?

Your doctor may do an eGFR test as part of a regular physical exam, or she might recommend it if you have signs of kidney problems. 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, like diabetes, high blood pressure, or heart disease. You may also get an eGFR test if other people in your family have had 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. He 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 see how much creatinine is 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, gender, and race.

In general, if the amount of protein in your urine is normal, an eGFR result 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.

What Can Affect the 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 those groups, talk with your doctor about how your results might be affected.

WebMD Medical Reference Reviewed by Minesh Khatri, MD on December 26, 2017

Sources

SOURCES:

National Kidney Foundation: “Glomerular Filtration Rate.”

National Institute of Diabetes and Digestive and Kidney Diseases: “Glomerular Filtration Rate.”

University of Rochester College of Medicine: “Glomerular Filtration Rate.”

University of North Carolina Kidney Center: “Glomerular Disease.”

American Journal of Roentgenology: “Kidney dimensions at sonography.”

Lab Tests Online: “Estimated Glomerular Filtration Rate,” “Creatinine.”

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