Glomerular Filtration Rate (GFR) Test
Reviewed
A glomerular filtration rate (GFR) test is a blood test that checks how well your kidneys are working by measuring how much blood your kidneys filter each minute. It is usually estimated (eGFR) using creatinine or cystatin C levels.
Preparation and collection
Your provider will let you know how to prepare. 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 tell your provider everything you're taking. Do not stop taking any medicine unless your provider tells you to stop.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- eGFR is an estimate and may not always reflect kidney damage.
- It's possible to have an abnormal result even if you don't have kidney damage.
- eGFR isn't a perfect test and may not always reflect kidney damage.
What to do next
If your eGFR is abnormal, your provider will explain what it means and may order additional tests such as a protein in urine test, microalbumin creatinine ratio, or BUN. If you are diagnosed with kidney disease, your provider will discuss steps to protect your kidneys and prevent further damage.
Evidence source
U.S. National Library of Medicine: Glomerular Filtration Rate (GFR) Test