Blood Urea Nitrogen (BUN) Test

Written by Rachel Nall | Published on July 25, 2012
Medically Reviewed by Brenda B. Spriggs, MD, MPH, FACP


Your kidneys are two bean-shaped organs responsible for filtering blood and waste. One method to determine just how well your kidneys are filtering is by measuring the waste product urea, using the blood urea nitrogen (BUN) test.

This fast, simple blood test also can tell your physician if you are eating too much protein, not drinking enough fluids, or your liver is not working properly.

What Does the BUN Test Tell Us?

When your liver filters proteins, it gives off urea nitrogen as a waste product. The kidneys are responsible for filtering the urea nitrogen out of your blood supply and into your urine. If a BUN test reveals higher-than-normal levels, your kidneys may not be working properly or as effectively as they should. The BUN test also can be used to determine how effective dialysis treatments are in acting as artificial kidneys.

If your physician orders a BUN test for you, he or she will also likely order a creatinine test. Both tests can indicate how well your kidneys are working. The typical ratio of BUN to creatinine in your blood is 10:1 to 20:1, according to the American Association for Clinical Chemistry. (AACC)

Your physician also may use the test to aid in the diagnosis of some of the following conditions:

  • liver damage
  • malnutrition
  • excess protein intake
  • poor circulation
  • dehydration
  • urinary tract obstruction
  • congestive heart failure
  • gastrointestinal bleeding

Pregnancy also can result in increased BUN levels.

How is the BUN Test Performed?

A BUN test is a simple blood test that does not require you to prepare or take any special precautions prior to the test. A medical provider will take a small sample of blood from a vein in your arm, which will then be sent to a laboratory for analysis.

Tell your doctor about any medications you may be taking, as some medications can affect your BUN levels. Your doctor will take this information into consideration when reviewing your test results.

If you’re taking the antibiotics chloramphenicol, typically used to treat eye and ear infections or streptomycin, used to treat more serious infections such as tuberculosis, your BUN levels may be lower than average.

Medications such as antibiotics and diuretics may increase your BUN levels. The following list represent just a few commonly prescribed medicines that may raise your BUN levels:

  • amphotericin B
  • carbamazepine
  • cephalosporins
  • furosemide
  • methotrexate
  • methyldopa
  • rifampin
  • spironolactone
  • tetracyclines
  • thiazide diuretics
  • vancomycin

How Are the Results of the BUN Test Interpreted?

Interpretations for your BUN test may vary based on the lab testing them. Average BUN values tend to range by gender and age. While reference levels have not been established for patients under the age of 12 months, it is generally accepted that infants have lower BUN levels, according to the Mayo Clinic. (Mayo) Accepted values include:

  • adult men: 8 to 24 mg/dL
  • adult women: 6 to 21 mg/dL

If your BUN results are higher than 50 mg/dL, this may indicate that your kidneys are not working properly. However, your physician will also take into account the medications you are taking that may increase BUN levels, including corticosteroids to relieve inflammation.

The BUN test is often one in a series of tests your physician will use to confirm a suspected diagnosis or recommend treatments.

Higher BUN levels can indicate:

  • congestive heart failure
  • gastrointestinal bleeding
  • heart attack
  • high protein levels
  • kidney disease
  • kidney failure
  • dehydration
  • obstruction in the urinary tract
  • shock

Lower BUN levels can indicate:

  • liver failure
  • malnutrition
  • insufficient dietary protein
  • excess hydration

What Are Side Effects of the BUN test?

You can typically return to your normal activities following a BUN test, if you’re not seeking care for an emergency medical condition. Always tell your physician if you have a medical condition that may affect bleeding or if you are taking medications, such as blood thinners, that may cause you to bleed more than expected.


Side effects associated with the BUN test include:

  • bleeding at the puncture site
  • bruising at the puncture site
  • feeling faint
  • accumulation of blood under the skin
  • infection at the puncture site

You should notify your medical provider if you experience unexpected or prolonged side effects.

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