Mayo Clinic Health Library

Blood urea nitrogen (BUN) test

Updated: 07-03-2013

Definition

A common blood test, the blood urea nitrogen (BUN) test reveals important information about how well your kidneys and liver are working. A BUN test measures the amount of urea nitrogen that's in your blood.

Here's how your body typically forms and gets rid of urea nitrogen:

  • Your liver produces ammonia — which contains nitrogen — after it breaks down proteins used by your body's cells.
  • The nitrogen combines with other elements, such as carbon, hydrogen and oxygen, to form urea, which is a chemical waste product.
  • The urea travels from your liver to your kidneys through your bloodstream.
  • Healthy kidneys filter urea and remove other waste products from your blood.
  • The filtered waste products leave your body through urine.

A BUN test can reveal whether your urea nitrogen levels are higher than normal, suggesting that your kidneys or liver may not be working properly.

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Why it's done

You may need a blood urea nitrogen test:

  • If your doctor suspects that you have kidney damage
  • If your kidney function needs to be evaluated
  • To help determine the effectiveness of dialysis treatment if you're receiving hemodialysis or peritoneal dialysis
  • As part of a blood test group to help diagnose a number of other conditions, such as liver damage, urinary tract obstruction, congestive heart failure or gastrointestinal bleeding — although an abnormal BUN test result alone doesn't confirm any of these conditions

If kidney problems are the main concern, when your blood is tested for urea nitrogen levels, it's likely it will also be tested for creatinine levels. Creatinine is another waste product that healthy kidneys filter out of your body through urine. High levels of creatinine may be a sign of kidney damage.

To get the best indication of how well your kidneys are removing waste from the blood, you may have a blood sample taken to calculate your estimated glomerular filtration rate (eGFR). The eGFR estimates the percentage of kidney function you have left.

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How you prepare

If your blood sample is being tested only for blood urea nitrogen, you can eat and drink normally before the test. If your blood sample will be used for additional tests, you may need to fast for a certain amount of time before the test. Your doctor will give you specific instructions.

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What you can expect

During the blood urea nitrogen test, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately.

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Results

Results of the blood urea nitrogen test are measured in milligrams per deciliter (mg/dL) in the United States and in millimoles per liter (mmol/L) internationally. In general, 7 to 20 mg/dL (2.5 to 7.1 mmol/L) is considered normal. But normal ranges may vary, depending on the reference range used by the lab, and your age. Ask your doctor to explain your results.

Urea nitrogen levels tend to increase with age. Infants have lower levels than other people do, and the range in children varies.

Generally, a high blood urea nitrogen level means your kidneys aren't working well. But elevated urea nitrogen can also be due to:

  • Urinary tract obstruction
  • Congestive heart failure or recent heart attack
  • Gastrointestinal bleeding
  • Dehydration, resulting from not drinking enough fluids or for other reasons
  • Shock
  • Severe burns
  • Certain medications, such as corticosteroids and some antibiotics
  • A high protein diet

If kidney damage is a concern, ask your doctor what factors may be contributing to the damage and what steps you can take to try to control them.

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