Blood urea nitrogen (BUN) test

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.


Why it's done How you prepare What you can expect Results

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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