Overview

A common blood test, the blood urea nitrogen (BUN) test reveals important information about how well your kidneys 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 may not be working properly.

Why it's done

You may need a BUN test:

  • If your doctor suspects that you have kidney disease or damage
  • If your kidney function needs to be evaluated, especially if you have a chronic condition such as diabetes or high blood pressure
  • 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, the creatinine levels in your blood will likely also be measured when your blood is tested for urea nitrogen levels. Creatinine is another waste product that healthy kidneys filter out of your body through urine. High levels of creatinine in your blood may be a sign of kidney damage.

Your doctor may also test how well your kidneys are removing waste from the blood. To do this, you may have a blood sample taken to calculate your estimated glomerular filtration rate (GFR). The GFR estimates the percentage of kidney function you have.

How you prepare

If your blood sample is being tested only for BUN, 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.

What you can expect

During the BUN 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.

Results

Results of the BUN test are measured in milligrams per deciliter (mg/dL) in the United States and in millimoles per liter (mmol/L) internationally. In general, around 6 to 24 mg/dL (2.1 to 8.5 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 BUN level means your kidneys aren't working well. But elevated BUN can also be due to:

  • Dehydration, resulting from not drinking enough fluids or for other reasons
  • Urinary tract obstruction
  • Congestive heart failure or recent heart attack
  • Gastrointestinal bleeding
  • Shock
  • Severe burns
  • Certain medications, such as 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.

By Mayo Clinic Staff