BUN

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BUN (blood urea nitrogen) is a test that measures the amount of urea nitrogen (a breakdown product of protein metabolism) in the blood.

Blood urea nitrogen

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:

The area is cleaned with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site if there is any continued bleeding.

There are no special preparations.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

The BUN test is a somewhat routine test used primarily to evaluate renal (kidney) function. The test is often performed on patients with many different diseases.

Urea is formed in the liver as the end product of protein metabolism. During digestion, protein is broken down to amino acids. Amino acids contain nitrogen, which is removed as NH4+ (ammonium ion), while the rest of the molecule is used to produce energy or other substances needed by the cell. The ammonia is combined with other small molecules to produce urea. The urea makes its way into the blood and it is ultimately eliminated in the urine by the kidneys.

Most renal diseases affect urea excretion so that BUN levels increase in the blood. Patients with dehydration or bleeding into the stomach and/or intestines may also have abnormal BUN levels. Numerous drugs also affect BUN by competing with it for elimination by the kidneys.

Review Date: 06/13/2005
Reviewed By: Debbie Cohen, M.D., Renal and Electrolyte Division, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.