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Blood Urea Nitrogen Test

Blood Urea Nitrogen Test



Blood urea nitrogen (BUN) is a chemical waste product of protein metabolism. Proteins are broken down into amino acids within the liver; these amino acids are metabolized, giving rise to nitrogen. Nitrogen is coupled with other molecules within the liver, producing the waste product urea that circulates in the bloodstream and goes to the kidneys. Healthy kidneys filter out this waste material from the blood. It passes into the urine and out of the body. Unhealthy kidneys, however, are unable to filter urea out of the blood. The urea remains circulating in the bloodstream, and blood urea nitrogen (BUN) levels rise as the liver continues to metabolize proteins.

The blood urea nitrogen level is used to predict how the kidneys are functioning. In many cases, the blood urea nitrogen level will begin to rise before a patient is even aware of any symptoms of kidney malfunction. High BUN levels indicate the need for further investigation into the possibility that kidney failure is ensuing. If a BUN level is elevated, then other tests such as a serum creatinine level or a 24-hour urine creatinine will be performed. Calculations involving serum and urine creatinine levels will give the creatinine clearance, a figure which reflects the capacity of the kidneys to filter small molecules out of the bloodstream.


A blood urea nitrogen level is usually drawn as part of a larger metabolic panel or screen. Other tests performed in this panel include electrolytes (sodium, potassium, chloride, and carbon dioxide), as well as calcium, glucose, and serum creatinine level. A blood urea nitrogen level is usually checked during a routine physical examination, as well as to evaluate acutely or chronically ill patients for the presence of kidney or liver disease, to monitor patients who have illnesses or who are taking medications that might affect the functioning of their kidneys, or to make sure that treatment for kidney disease (including hemodialysis or peritoneal dialysis) is effective.


Blood urea nitrogen levels can be affected by a number of medications. Patients who are on these medications should inform their doctor, so that test results can be interpreted appropriately. Medications that may affect blood urea nitrogen levels include diuretics, amphotericin B, nafcillin, aminoglycosides, kanamycin, tobramycin, steroid medications, tetracycline antibiotics, and chloramphenicol. Additionally, if the blood urea nitrogen level is going to be used in calculations with serum or urine creatinine levels to evaluate kidney functioning, results may be skewed by the following medications: methyldopa, trimethoprim, vitamin C, cimetidine, certain diuretics, cephalosporin antibiotics, phenytoin, captopril, quinine, quinidine, and procainamide.

Patients who are taking anticoagulant medications should inform their healthcare practitioner since this may increase their chance of bleeding or bruising after a blood test.


This test requires serum to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw serum). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The serum is collected in vacuum


Creatine— Creatine is a substance produced from protein and stored in the muscles. Creatine is a source for energy, allowing muscle contraction to take place. Some creatine is converted to creatinine, and enters the bloodstream, where it is filtered out by healthy kidneys and leaves the body in the urine. When the kidneys are not functioning properly, creatinine levels in the blood become abnormally elevated.

Creatinine— Creatinine is a chemical waste product that is produced by the muscles. Creatinine enters the bloodstream and goes to the kidneys. Healthy kidneys filter out this waste material from the blood. It passes into the urine and out of the body. Unhealthy kidneys, however, are unable to filter out the creatinine from the blood. The creatinine remains circulating in the bloodstream, and levels rise as the muscles continue to produce more and more.

Urine creatinine level— A value obtained by testing a 24-hour collection of urine for the amount of creatinine present.

tubes. After collection, the needle is withdrawn, and pressure is kept on the serum draw site to stop any bleeding and decrease bruising. A bandage is then applied.


In the 24-48 hours prior to a blood urea nitrogen level, patients should be advised to limit the amount of protein they ingest. Because urea is a waste product of protein metabolism, ingesting more than eight ounces of meat (particularly beef) or other protein sources in the 24 hours prior to the blood urea nitrogen level is performed may affect the results.


As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a serum test, and they should be encouraged to lie down and rest until they feel better.


Basic serum tests, such as blood urea nitrogen levels, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.


A normal blood urea nitrogen level is 10-20 milligrams per deciliter (mg/dL) or 3.6-7.1 millimoles per liter (mmol/L). Women and children metabolize protein slightly differently than do men, so their BUN levels may normally be lower than those of men. BUN levels also regularly rise with age, and

High levels

High blood urea nitrogen levels suggest that the kidneys are suffering from damage or disease. Kidneys can be damaged by severe infections, shock, cancer, dehydration, high blood pressure, diabetes, or conditions that limit the blood flow reaching the kidneys (such as heart attack, stress, shock, congestive heart failure, or severe burns). High blood urea nitrogen levels can also occur when the urinary tract is blocked (by a kidney stone or tumor). Other causes of increased BUN include excess Addison’s disease, dietary intake of protein, bleeding in the gastrointestinal tract (resulting in the metabolism of these blood proteins generating increased urea), tissue damage that increases protein levels that reach the liver (such as may occur with very severe burns), or increases in the rate of protein metabolism in the body. BUN levels may also be elevated during a completely normal pregnancy.

Low levels

Low blood urea nitrogen levels are not diagnostic; however, they may reflect the presence of conditions such as overhydration, poor nutrition, liver disease, or pregnancy.



Brenner, B. M., and F. C. Rector, eds. Brenner & Rector’s The Kidney, 7th ed. Philadelphia: Saunders, 2004.

Goldman L., D. Ausiello, eds. Cecil Textbook of Internal Medicine, 23rd ed. Philadelphia: Saunders, 2007.

McPherson R. A., and M. R. Pincus, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods, 21st ed. Philadelphia: Saunders, 2006.


Medical Encyclopedia. Medline Plus. U.S. National Library of Medicine and the National Institutes of Health. January 2, 2008. (February 10, 2008).


American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400,

Rosalyn Carson-DeWitt, M.D.

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