Your kidneys are the body’s main filtration system, removing waste products from your blood and releasing them via your urine. The small filters inside your kidneys are known as glomeruli. If your kidneys are not working properly, your glomeruli will not filter as quickly. Your doctor may order a glomerular filtration rate (GFR) test if he or she suspects your kidneys may not be working properly. This simple blood test involves few side effects.
Since the GFR test can indicate how well your kidneys are functioning, the test is ordered when you have symptoms related to kidney disease or when your doctor wants to test the effectiveness of a particular treatment. According to the American Association for Clinical Chemistry, examples of kidney disease symptoms include unexplained body swelling, foamy urine, difficulty urinating, and mid-back pain. (AACC)
Early intervention is vital to preventing further kidney damage, which is why your physician may recommend a GFR test if you have the following conditions:
- recurring urinary tract infections
- heart disease
- urinary blockage
If your doctor has diagnosed you with kidney disease, the GFR test can help diagnose how well your kidneys are functioning.
If you have a family history of kidney disease, your doctor may want to run a GFR test to get a sense of the current state of your kidneys.
The GFR test involves taking a blood sample to test for creatinine. This substance is a waste product your kidneys filter and release in the urine. If you have high amounts of creatinine in your blood, this can indicate that your kidneys are not filtering adequately.
Your doctor or nurse will obtain a sample by drawing blood from your arm. Because GFR is calculated using a specific formula, whoever draws the blood will also ask you relevant information, such as:
A laboratory specialist will take these factors into account to calculate the most accurate GFR.
The GFR test is sometimes known as the estimated GFR or eGFR test because several calculations are required to arrive at your final GFR. This is why the GFR test is considered an indirect measurement of how well your kidneys may be functioning.
The following are typical GFRs, according to the National Kidney Foundation and American Association for Clinical Chemistry:
- healthy adults: 90 mL/min/1.73 m2 or higher
- children and the elderly: 60 to 89 mL/min/1.73 m2
- early kidney disease: 60 to 89 mL/min/1.73 m2 for three months or more along with signs of kidney damage
- chronic kidney disease: less than 60 mL/min/1.73 m2 for three months or more
- kidney failure: less than 15 to 60 mL/min/1.73 m2 (NKF)
Note that many different calculations are available for testing GFR. Your results may vary from the numbers listed above, depending on the laboratory’s testing ranges. Your doctor will likely recommend taking your GFR over the course of several months to establish a pattern.
Certain medications can affect your creatinine levels. You should notify your physician if you are taking any of the following medications:
- cephalosporin or aminoglycoside antibiotics
Because the GFR requires taking a small amount of blood, it is not associated with major side effects. You can typically resume activity immediately following the test. However, you may experience some throbbing or bruising at the puncture site. If you have unexplained bleeding or severe discomfort following the test, report this to your medical provider.