Glomerular filtration rate (GFR) is a test used to check how well the kidneys are working. Specifically, it estimates how much blood passes through the tiny filters in the kidneys, called glomeruli, each minute.
A blood sample is needed. For information on how this is done, see: Venipuncture
The blood sample is sent to a lab, where the creatinine level is tested. The lab specialist uses combines your creatinine level with several other factors to estimate your glomerular filtration rate (GFR). Different formulas are used for adults and children. The formula will include some or all of the following:
How to prepare for the test
Certain drugs can interfere with test results. Tell your doctor about all medicines you are taking, including over-the-counter ones.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there might be some throbbing.
Why the test is performed
The GFR test measures how well your kidneys are filtering a waste called creatinine, which is produced by the muscles. When the kidneys aren't working as well as they should, creatinine builds up in the blood.
Your doctor may order this test if there are signs that your kidneys are not working properly. It may also be done to see how far kidney disease has progressed.
The test is recommended for people with chronic kidney disease and those at risk for it due to:
According to the National Kidney Foundation, normal results range from 90 - 120 mL/min/1.73 m2. Older people will have lower normal GFR levels, because GFR decreases with age.
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
Levels below 60 mL/min/1.73 m2 for 3 or more months are a sign of chronic kidney disease. A GFR result below 15 mL/min/1.73 m2 is a sign of kidney failure and requires immediate medical attention.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but can include:
Fainting or feeling lightheaded
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
The creatinine clearance test, which involves a 24-hour urine collection, can also provide an estimate of kidney function. However, this method is not always accurate.
GFR can increase during pregnancy.
Bazari H. Approach to the patient with renal disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 115.
Israni AK, Kasiske BL. Laboratory assessment of kidney disease: clearance, urinalysis, and kidney biopsy. In: Brenner BM, ed. Brenner and Rector's The Kidney. 8th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 23.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Herbert Y. Lin, MD, PHD, Nephrologist, Massachusetts General Hospital; Associate Professor of Medicine, Harvard Medical School. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.