What Is a Blood Urea Nitrogen Test?

Medically Reviewed by Nayana Ambardekar, MD on May 02, 2023
5 min read

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.

Your doctor may order a BUN test as part of a routine checkup. It may be one of several blood tests that you get.

If you have a kidney condition, the test is a way to check what your BUN levels are before you start a medication or treatment. Also, it’s standard for a BUN test to be given when you’re in the hospital for certain conditions.

If your doctor suspects you may be getting kidney problems, they may order the BUN test.

Tell your doctor if you have the following symptoms, which can be signs that something is wrong with your kidneys:

● A change in how much you urinate

● Pee that is foamy, bloody, discolored, or brown

Pain while you pee

● Swelling in your arms, hands, legs, ankles, around your eyes, face, or abdomen

Restless legs during sleep

● Pain in the mid-back where kidneys are located

● You’re tired all the time

Before the blood test, tell your doctor what medications you’re taking. If any of them might alter the test result, your doctor may ask you to stop taking them for a period of time.

If you’re only getting a BUN test, you can eat and drink. But if you’re getting other blood tests, your doctor may give you directions that may include fasting before the test.

A lab tech will take a sample of your blood from a vein in your arm or the back of your hand. You may feel a slight sting when the needle pricks through your skin.

It may feel a little bit sore afterward, but you can go straight back to your everyday activities.

Your doctor’s office will send the blood sample to a lab to be analyzed. You should get the results in a few days, depending on how fast the lab and your doctor’s office can work.

Your result will be a number that measures how much BUN is in your blood. The range considered normal is between 7 to 20 milligrams per deciliter. (A milligram is a very tiny amount -- more than 28,000 to an ounce, and a deciliter is equal to about 3.4 ounces).

If your test results are not in that range, talk to your doctor.

Several things can affect your BUN test results, so having a BUN level that is lower or higher than the normal range doesn’t always mean there is a problem.

Things that affect your BUN level might include:

● High-protein diet (may cause high BUN levels)

● Low-protein diet  (may cause low BUN levels)

● Several medications, including steroids and antibiotics (increased or decreased BUN levels)


● Aging

High BUN levels can also indicate various problems with your kidneys. Talk to your doctor about what could be causing the problem and plan your next steps.

High levels can also indicate the following:


● Urinary tract obstruction (blockage from being able to pee)

Congestive heart failure (when your heart doesn’t pump blood to your body like it should)

● Shock

Burn injuries


Heart attack

Gastrointestinal bleeding (bleeding in your digestive tracts, such as your stomach, intestines, or esophagus)

Low BUN levels are rare. If you have low BUN levels, it could indicate:

Liver disease

● Malnutrition (when your diet doesn’t have enough nutrients or your body can’t take them in well)

Overhydration (having too much fluid)

But a BUN test is not a way to diagnose these issues, so more tests may be needed

Your doctor may also order a creatinine test, which is another blood test that also checks your kidney health. This is because the BUN level by itself doesn’t always reveal much.

When your BUN levels are compared with your creatinine levels, it gives a fuller picture of what’s happening with your kidneys. This is known as the BUN/Creatinine ratio.

Creatinine is a waste product from your muscles that is also filtered by your kidneys. Like BUN, high levels of creatinine could mean there is a lot of waste product that hasn’t been removed by the kidneys.

The ideal ratio of BUN to creatinine falls between 10-to-1 and 20-to-1.

Having a ratio above this range could mean you may not be getting enough blood flow to your kidneys, and could have conditions such as congestive heart failure, dehydration, or gastrointestinal bleeding.

A ratio below the normal range could mean liver disease or malnutrition.

Show Sources


American Association for Clinical Chemistry: "Blood Urea Nitrogen."

University of Rochester Medical Center: "Blood Urea Nitrogen."

National Kidney Foundation: "Tests to Measure Kidney Function."

Mayo Clinic: "Blood Urea Nitrogen Test" and "Heart Failure."

Scripps Health Foundation: "BUN -- Blood Test."

U.S. National Library of Medicine: "Gastrointestinal Bleeding."

American Association for Clinical Chemistry: "Creatinine."

National Health Service (U.K.): "Malnutrition."


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