What Is a Blood Urea Nitrogen Test?

Medically Reviewed by Jabeen Begum, MD on October 30, 2023
6 min read

Doctors order a blood urea nitrogen (BUN) test to measure the amount of urea nitrogen in your blood. It helps them see how well your kidneys are working.


Urea nitrogen is a normal waste product created in your liver as it breaks down certain proteins found in your food. Urea nitrogen travels through the blood to your kidneys, which filter out all but a small amount. Urea nitrogen exits from your body when you pee.

When your kidneys are healthy, they remove BUN, usually leaving only a small amount in your blood. When your kidneys are not healthy, they leave more BUN behind. The BUN blood test measures the amount of urea nitrogen in your blood. If your levels are outside the normal range, this might mean either your liver or kidneys are not working well.

Your doctor may order a BUN test as part of a routine checkup when a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP) blood test is taken.

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. It’s also standard to perform a BUN test when you’re in the hospital for certain conditions.

If you're at higher risk for kidney disease because you have a family history of it, your doctor might order a BUN test for you.

The following conditions may also put you at risk:

  • High blood pressure
  • Heart disease
  • Diabetes

Tell your doctor if you have any of the following symptoms, which can indicate 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
  • Mid-back pain around the kidneys
  • You’re tired all the time

How do I prepare for a BUN test?

Before the blood test, tell your doctor what medications you’re taking. If any of them might affect 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 include fasting before the test.

What happens during a BUN 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 your skin.

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

Risks of a blood urea nitrogen test

Blood tests draw a small amount of blood, and there are usually few or mild side effects. You may have pain, bruising, or slight bleeding at the site where the blood is drawn. Some people may even feel lightheaded or faint. It doesn't happen often, but the site could get infected.

The normal range for a BUN test depends on your age and sex, or the one you were assigned at birth.

Normal BUN levels

Your result will be a number that measures how much BUN is in your blood. The range considered normal is 7-20 milligrams per deciliter. (A milligram is a very tiny amount—with over 28,000 milligrams in 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 an abnormal BUN level doesn’t always mean there's a problem.

BUN normal values by age and sex

 Male or Assigned Male at BirthFemale or Assigned Female at Birth
Ages 1 to 177 to 20 mg/dL7 to 20 mg/dL
Ages 18 and older8 to 24 mg/dL6 to 21 mg/dL

High BUN

A high BUN test result may mean your kidneys aren't working well. Discuss with your doctor what could be causing the problem and plan your next steps.

Other conditions that may affect your BUN level might include:


Low BUN levels are rare. It's more likely if you have a small body. It could also point to:

A BUN test by itself can't diagnose these issues, so more tests may be needed.

If your BUN levels are high, your doctor may order more tests to measure your kidney function.

Creatinine test

A creatinine test is another blood test that checks your kidney health. You'd get it because the BUN level by itself doesn’t always reveal much.

When your BUN levels are compared to 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's also filtered by your kidneys. Like BUN, high levels of creatinine could mean there's a lot of waste product that hasn’t been removed by the kidneys.

BUN to creatinine ratio

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.

Glomerular filtration rate (GFR) test

The GFR test measures how well your kidneys filter waste from your blood. It will tell your doctor how much kidney function you have. A very low number means kidney failure, which means you'll need dialysis (a treatment that helps your kidneys remove waste or fluids from your blood) or a kidney transplant.


There's no magic pill for treating high BUN levels. Doctors try to find out what's harming your kidney function and treat that condition. You can make changes to your diet to help lower BUN levels.

Drink more fluids. For men or those assigned male at birth, that means about 3 liters (13 cups) of fluid each day. Women and those assigned female at birth should take in about 9 cups of fluid a day. (If you already have serious kidney disease or are on dialysis, too much fluid can cause problems, so follow your doctor's recommendation.)

A diet low in protein can also help lower BUN levels. Eat healthy foods, such as:

  • Pasta, oats, and rice
  • Fruits including bananas, berries, and apples 
  • Leafy green and other vegetables
  • Avocados, nuts, and healthy fats

You should limit high-protein foods such as meats and dairy products. To reduce your BUN levels, don't eat more than 20-30 grams of protein a day.

Having a low BUN level is usually not serious. However, if you want to raise your levels, you should eat more protein and improve the health of your liver by drinking less alcohol and losing weight.

If your test results show high BUN levels, talk with your doctor about whether you'll need more tests to check your kidney function. There are no medications to treat high BUN levels. However, you can make changes to your diet that may help lower or raise BUN levels.