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Also known as: Urea Nitrogen; Urea
Formal name: Blood Urea Nitrogen

At a Glance

Why Get Tested?

To evaluate kidney function; 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 or when you are acutely or chronically ill with a condition that may cause or be worsened by kidney dysfunction

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

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

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 bloodstream and is carried to the kidneys, where it is filtered out of the blood and excreted in 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 decreased amounts are excreted by the kidneys, then urea concentrations will rise. If significant liver damage or disease inhibits the production of urea, then BUN concentrations may fall.

How is the sample collected for testing?

A blood sample is drawn from a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Lerma, E. (Updated 2012 October 30). Blood Urea Nitrogen (BUN). Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2073979-overview through http://emedicine.medscape.com. Accessed November 2012.

Dugdale, D. (2011 May 30). BUN - blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003474.htm. Accessed November 2012.

(© 1995-2012). Blood Urea Nitrogen (BUN), Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/81793 through http://www.mayomedicallaboratories.com. Accessed November 2012.

(Updated 2012 March 23). The Kidneys and How They Work. National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC) [On-line information]. Available online at http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/ through http://kidney.niddk.nih.gov. Accessed November 2012.

McMillan, J. (Modified 2010 January). Chronic Kidney Disease. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed November 2012.

Arora, P. (Updated 2012 March 28). Chronic Kidney Disease. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/238798-overview through http://emedicine.medscape.com. Accessed November 2012.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 993-995.

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 through http://kidney.niddk.nih.gov.

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 through http://www.annals.org.

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 through http://www.aafp.org.

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

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 through http://www.kidney.niddk.nih.gov. 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 through http://www.urologyhealth.org. Accessed November 2008.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pp. 312-313.

Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006 Pp. 525-527.