What is a glomerular filtration rate test?

Your kidneys are your body’s main filtration system. They remove waste products from your blood and excrete them via your urine. Glomeruli are the small filters inside your kidneys. If your kidneys aren’t working properly, your glomeruli won’t filter as efficiently. Your doctor may order a glomerular filtration rate (GFR) test if they suspect your kidneys aren’t working properly. This is a simple blood test.

The GFR test can indicate how well your kidneys are functioning. Your doctor may order the test if you have symptoms related to kidney disease or if they want 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
  • mid-back pain

Early intervention is vital to preventing further kidney damage. Your doctor may recommend a GFR test if you’re taking medications that can affect your kidney function or if you have any of the following conditions:

  • diabetes
  • recurring urinary tract infections
  • hypertension
  • heart disease
  • difficulty with urination
  • blood in the urine
  • kidney stones
  • polycystic kidney disease
  • kidney failure

If you’ve been diagnosed with kidney disease, the GFR test can help determine 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.

A GFR test is a simple blood test that doesn’t require you to do anything to prepare.

A blood sample will be taken by drawing blood from your arm. Because there’s a specific formula used to calculate GFR, you may also need to provide your:

  • age
  • sex
  • race
  • height
  • weight

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 necessary to arrive at your final GFR. This is why the GFR test is an indirect measurement of how well your kidneys may be functioning.

According to the National Kidney Foundation (NKF), the lower your GFR results, the more damage your kidneys have. Your doctor can use your GFR to determine the level of kidney damage you have. This is also known as the stage of your kidney damage. The NKF states that the stages of kidney damage are:

  • stage 1: minimal or no loss of kidney function (GFR of 90 or above)
  • stage 2: mild loss of kidney function (GFR of 60 to 89)
  • stage 3: moderate loss of kidney function (GFR of 30 to 59)
  • stage 4: severe loss of kidney function (GFR of 15 to 29)
  • stage 5: kidney failure (GFR of 15 or below)

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. When getting your GFR, your doctor will also get a creatinine level. You should notify your doctor if you’re taking any of the following medications:

  • cephalosporin antibiotics
  • aminoglycoside antibiotics
  • flucytosine
  • cisplatin
  • cimetidine
  • trimethoprim
  • ibuprofen if you’re an older adult

The GFR test only requires taking a small amount of blood. It typically doesn’t cause any major side effects. You can resume activity immediately after the test. However, you may experience some throbbing or bruising at the puncture site. Tell your doctor if you have unexplained bleeding or severe discomfort following the test.

A GFR test is a simple blood test that doesn’t require you to do anything to prepare. The test will measure the amount of the waste product creatinine in your blood. The test is done to determine how well your kidneys are working. Based on your GFR results, your doctor can determine how much, if any, kidney damage you have.