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Blood Urea Nitrogen

What Is Urea Nitrogen?

Urea nitrogen is a waste product made by the liver. It passes into the blood. The kidneys then filter urea out of the blood. Next, it is passed out of the body in urine.

Reason for the Test

A blood urea nitrogen (BUN) test is done to measure the amount of urea in the blood. It may be done:

  • As a screening test
  • To look for signs of kidney problems
  • To monitor kidney problems

Type of Sample Taken

A blood sample will be taken from a vein in the arm.

Prior to Collecting the Sample

Your doctor may ask you to stop taking certain medicines ahead of the test.

During the Sample Collection

You will be asked to sit. An area on your arm will be cleaned with a wipe. A large band will be tied around your arm. The needle will be put in a vein. A tube will collect the blood from the needle. The band on your arm will be taken off. After the blood is collected, the needle will be removed. Gauze will be held on the site to help stop bleeding. You may also be given a bandage. This test takes about 5 to 10 minutes.

After Collecting the Sample

After the blood sample is taken, you may need to stay seated for 10 to 15 minutes. If you are lightheaded, you may need to stay seated longer. When you feel better, you can leave.

A bit of blood may ooze from the vein beneath the skin. It will cause a bruise. A bruise will usually fade in a day or two.

Call your doctor right away if you have redness, swelling, lasting bleeding, or pain.

Results

It will take a few days to get your test results.

High BUN levels may be caused by:

  • Problems with how the kidneys are working
  • Lower blood flow in the kidneys
  • Stress
  • A block in the urinary tract
  • Excess protein breakdown or intake
  • Muscle injury or disease
  • Gastrointestinal bleeding
  • Heart failure
  • Heart attack
  • Shock

Low BUN levels may be caused by:

  • Higher amounts of urine being made
  • Severe liver damage
  • Higher amounts of new proteins being made
  • Low protein and high carbohydrate diets
  • Nephrotic syndrome
  • Syndrome of inappropriate antidiuretic hormone secretion (SIADS)
  • Inherited hyperammonemias

Talk to your doctor about your test results. A test may point to an illness that you do not have. It can also miss an illness that you may have. The doctor will check your symptoms and all test results before making a diagnosis.

REFERENCES:

Blood urea nitrogen (BUN). Lab Tests Online—American Association for Clinical Chemistry website. Available at: https://labtestsonline.org/tests/blood-urea-nitrogen-bun. Accessed May 12, 2021.

Blood urea nitrogen measurement. EBSCO DynaMed website. Available at: https://www.dynamed.com/lab-monograph/blood-urea-nitrogen-measurement. Accessed May 12, 2021.

Last reviewed March 2021 by EBSCO Medical Review Board Daniel A. Ostrovsky, MD