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

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.