What is the blood urea nitrogen (BUN) test?

A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Levels of urea nitrogen is one marker on how well your kidneys are working. This is a simple test done by drawing blood out of your body through a vein in your arm.

Urea is a waste product formed in the liver that travels through your blood to the kidneys, which then filters it out of the blood. It is then carried out of your body through urine. A small amount of urea in your blood is normal because this process is ongoing. Too much urea shows that it is not being filtered out properly and may indicate a possible problem with the kidneys.

Why is blood urea nitrogen (BUN) test done?

A BUN test is a routine test ordered by your doctor during your checkup as part of a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP). It will be done if you are admitted to an emergency room or during a regular hospital stay.

The BUN test may also be ordered as a precaution if you have risk factors for kidney disease. Early kidney disease doesn’t have symptoms, but the following factors can put you at higher risk:

The BUN test is often ordered if the doctor suspects your symptoms are signs of kidney disease. These include:

  • Fatigue
  • Frequent urination or not often enough
  • Urine that is discolored or unusual (bloody, foamy, coffee-colored)
  • Swelling around the eyes or on the face, belly, arms, legs, or feet
  • High blood pressure
  • Nausea or vomiting

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