BUN

Also known as: Urea nitrogen; Urea

Formal name: Blood Urea Nitrogen

Related tests: Creatinine; Creatinine Clearance; eGFR; CMP; BMP; Urinalysis; Microalbumin

At a Glance

Why Get Tested?

To evaluate kidney function or 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 the arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

This test measures the amount of urea nitrogen in the blood. Nitrogen, in the form of ammonia, is produced in the liver when protein is broken into its component parts (amino acids) and metabolized. The nitrogen combines with other molecules in the liver to form the waste product urea. The urea is then released into the bloodstream and 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

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 patients 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 basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).

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When is it ordered?

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

When someone has non-specific complaints
As part of a routine testing panel
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 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:

At regular intervals to monitor kidney function in patients with chronic diseases or conditions such as diabetes, congestive heart failure, and myocardial infarction (heart attack)
At regular intervals to monitor kidney function and treatment in patients with known kidney disease
Prior to and during certain drug treatments to monitor kidney function
At regular intervals to monitor the effectiveness of dialysis.

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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 patient is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.

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

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

NOTE: The result of your BUN test is measured by your doctor against a reference range for the test to determine whether the result is “normal” (it is within the range of numbers), high (it is above the high end of the range), or low (it is below the low end of the range). Because there can be many variables that affect the determination of the reference range, the reference range for this test is specific to the lab where your test sample is analyzed. For this reason, the lab is required to report your results with an accompanying reference range. Typically, your doctor will have sufficient familiarity with the lab and your medical history to interpret the results appropriately

While there is no such thing as a “standard” reference range for BUN, most labs will report a similar, though maybe not exactly the same, set of numbers as that included in medical textbooks or found elsewhere online. For this reason, we recommend that you talk with your doctor about your lab results. For general guidance only, we are providing the reference range for this test from the classic medical text, Tietz Textbook of Clinical Chemistry and Molecular Diagnostics.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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Is there anything else I should know?

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

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Common Questions

1. What other tests are used with BUN 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 doctor may also order electrolyte tests such as sodium and potassium, or calcium to help understand how your kidneys are functioning.

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2. 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. Levels are also slightly higher in men than women.

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3. What is a BUN/Creatinine ratio?

Occasionally, a doctor will look at the ratio between a person’s BUN and blood creatinine to help them 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 . 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.

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Ask a Laboratory Scientist

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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

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.

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.