Also Known As
BUN
Urea Nitrogen
Urea
Formal Name
Blood Urea Nitrogen
This article was last reviewed on
This article waslast modified on July 11, 2018.
At a Glance
Why Get Tested?

To evaluate the health of your kidneys; to help diagnose kidney disease; to monitor the effectiveness of dialysis and other treatments related to kidney disease or damage

When To Get Tested?

As part of a routine comprehensive or basic metabolic panel; when you have signs and symptoms that may be due to kidney disease or you have a condition that may cause or be worsened by kidney dysfunction; at regular intervals when you are being treated for kidney disease or damage

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

What is being tested?

Urea is a waste product formed in the liver when protein is metabolized into its component parts (amino acids) . This process produces ammonia, which is then converted into the less toxic waste product urea. This test measures the amount of urea nitrogen in the blood.

Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the "transport method" used by the body to rid itself of excess nitrogen. Urea is released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood.

Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or if the kidneys are not working properly and have difficulty filtering wastes out of the blood, then urea concentrations will rise in the blood. If significant liver damage or disease inhibits the production of urea, then BUN concentrations may fall.

Accordion Title
Common Questions
  • How is it used?

    The blood urea nitrogen or BUN test is primarily used, along with the creatinine test, to evaluate kidney function in a wide range of circumstances, to help diagnose kidney disease, and to monitor people with acute or chronic kidney dysfunction or failure. It also may be used to evaluate a person's general health status when ordered as part of a renal panel, basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).

    Urea is a waste product formed in the liver when protein is metabolized. Urea is released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.

    The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called a glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what can be used by the body. The remaining waste forms urine.

    If the creatinine and BUN tests are found to be abnormal or if someone has an underlying disease that is known to affect the kidneys, such as diabetes or high blood pressure, then creatinine and BUN tests may be used to monitor for kidney dysfunction and the effectiveness of treatment. Blood creatinine and BUN tests may also be ordered to evaluate kidney function prior to some procedures, such as a CT (computed tomography) scan, that may require the use of drugs that can damage the kidneys.

  • When is it ordered?

    BUN is part of both the BMP and CMP, groups of tests that are widely used:

    • As part of a routine health checkup
    • To check how the kidneys are functioning before starting to take certain drug therapies
    • When an acutely ill person comes to the emergency room and/or is admitted to the hospital
    • During a hospital stay

    BUN is often ordered with creatinine or renal panel when kidney problems are suspected. Some signs and symptoms of kidney dysfunction include:

    • Fatigue, lack of concentration, poor appetite, or trouble sleeping
    • Swelling or puffiness (edema), particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
    • Urine that is foamy, bloody, or coffee-colored
    • A decrease in the amount of urine
    • Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
    • Mid-back pain (flank), below the ribs, near where the kidneys are located
    • High blood pressure

    BUN also may be ordered:

  • What does the test result mean?

    Increased BUN levels suggest impaired kidney function. This may be due to acute or chronic kidney disease, damage, or failure. It may also be due to a condition that results in decreased blood flow to the kidneys, such as congestive heart failure, shock, stress, recent heart attack, or severe burns, to conditions that cause obstruction of urine flow, or to dehydration.

    BUN concentrations may be elevated when there is excessive protein breakdown (catabolism), significantly increased protein in the diet, or gastrointestinal bleeding (because of the proteins present in the blood).

    Low BUN levels are not common and are not usually a cause for concern. They may be seen in severe liver disease, malnutrition, and sometimes when a person is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.

    If one kidney is fully functional, BUN concentrations may be normal even when significant dysfunction is present in the other kidney.

  • Is there anything else I should know?

    BUN levels can increase with the amount of protein in the diet. High-protein diets may cause abnormally high BUN levels while very low-protein diets can cause an abnormally low BUN.

    A wide variety of drugs can cause an increase in BUN. Drugs that can decrease BUN include chloramphenicol and streptomycin. Inform your healthcare provider of any medications you are taking.

    Both decreased and increased BUN concentrations may be seen during a normal pregnancy.

  • What is a BUN/Creatinine ratio?

    Occasionally, a health practitioner will look at the ratio between a person's BUN and blood creatinine to help determine what is causing these concentrations to be higher than normal. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as congestive heart failure or dehydration. It may also be seen with increased protein, from gastrointestinal bleeding, or increased protein in the diet. The ratio may be decreased with liver disease (due to decrease in the formation of urea) and malnutrition.

  • What other tests are used with the blood urea nitrogen test to check how my kidneys are functioning?

    BUN and creatinine are the primary tests used to check how well the kidneys are able to filter waste products from your blood.  Your healthcare provider may also order a renal panel or electrolyte tests, such as sodium and potassium, or calcium to help understand how your kidneys are functioning.

  • How does BUN change with age?

    BUN levels increase with age.  BUN levels in very young babies are about 2/3 of the levels found in healthy young adults, while levels in adults over 60 years of age are slightly higher than younger adults.

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

BUN Reference Range

The reference ranges1 provided here represent a theoretical guideline that should not be used to interpret your test results. Some variation is likely between these numbers and the reference range reported by the lab that ran your test. Please consult your healthcare provider.

Age Conventional Units2 SI Units3
0-18 years Not available due to wide variability. See child's lab report for reference range.
Adult 6-20 mg/dL 2.1-7.1 mmol/L
>60 years 8-23 mg/dL 2.9-8.2 mmol/L

1 from Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 5th edition, St. Louis: Elsevier Saunders; 2011.

2 Conventional Units are typically used for reporting results in U.S. labs

3 SI Units are used to report lab results outside of the U.S.

View Sources

Sources Used in Current Review

Lerma, E. (2014 March 6, Updated). Blood Urea Nitrogen (BUN). Medscape Drugs & Diseases. [On-line information]. Available online at http://emedicine.medscape.com/article/2073979-overview. Accessed 07/18/15.

Salazar, J. (2014). Overview of Urea and Creatinine. Medscape Multispecialty from Lab Med. 2014;45(1):e19-e20. [On-line information]. Available online at http://www.medscape.com/viewarticle/823420. Accessed 07/18/15.

Dugdale, D. (2013 April 29). BUN - blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003474.htm. Accessed 07/18/15.

Delgado, J. and Straseski, J. (2015 May). Renal Function Markers - Kidney Disease. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/RenalFunctionMarkers.html. Accessed 07/18/15.

(© 1995–2015). Blood Urea Nitrogen (BUN), Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/81793. Accessed 07/18/15.

Pagana, K. D., Pagana, T. J., and Pagana, T. N. (© 2015). Mosby's Diagnostic & Laboratory Test Reference 12th Edition: Mosby, Inc., Saint Louis, MO. Pp 946-948.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(2002 March). Medical Tests of Kidney Function. National Kidney and Urologic Diseases Information Clearinghouse, NIH Publication No. 02–4623 [On-line information]. Available online at http://kidney.niddk.nih.gov/kudiseases/pubs/kidneytests/index.htm.

Esson, M. and Schrier, R. (2002). Diagnosis and Treatment of Acute Tubular Necrosis. Ann Intern Med 2002;137:744-752 [On-line journal]. PDF available for download at http://www.annals.org/cgi/reprint/137/9/744.pdf.

Agha, Irfan (2003 August 7, Updated). BUN. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003474.htm.

Agrawal, M. and Swartz, R. (2000 April 1). Acute Renal Failure. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20000401/2077.html.

Physician’s Reference Laboratory: Kidney Function Panel. Available online at http://www.prlnet.com/Kidney.htm.

August 2007) National Kidney and Urological Disease Information Clearinghouse (NKUDIC). Your Kidneys and How They Work. Available online at http://www.kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/index.htm#rate. Accessed November 2008.

(Update May 15, 2007) MedlinePlus Medical Encyclopedia. BUN. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003474.htm. Accessed November 2008.

American Urological Association. UrologyHealth.org, Kidney (renal) Failure. Available online at http://www.urologyhealth.org/adult/index.cfm?cat=02&topic=120#top. Accessed November 2008.

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