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.

Why this test is done

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

How to prepare

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.

Sample type

Blood

Other names for this test

estimated GFR, eGFR, calculated glomerular filtration rate, cGFR