What Is a Blood Urea Nitrogen Test?

Your doctor may order a blood urea nitrogen test as part of a routine health screening. It helps them see how well your kidneys are working.

Urea nitrogen is a normal waste product that your body creates after you eat. Your liver breaks down the proteins in your food — and while it does that, it creates blood urea nitrogen, also known as BUN. Your liver releases the substance into the blood, and it eventually ends up in your kidneys.

When your kidneys are healthy, they remove the BUN, usually leaving a small amount of it in the blood. But for the most part, your kidneys get rid of it by flushing it out of your body through urine.

When your kidneys are not healthy, they have trouble removing BUN and leave more of it in your blood.

The blood urea nitrogen test, which is also called a BUN or serum BUN test, measures how much of the waste product you have in your blood. If your levels are off the normal range, this could mean that either your kidneys or your liver may not be working properly.