Normal BUN, blood urea nitrogen, levels indicate proper functioning of the liver and kidneys, according to Mayo Clinic. The liver produces nitrogen when it breaks down protein, which along with other chemicals forms urea. Urea enters the bloodstream and moves to the kidneys, where it is filtered from the blood.
A doctor may order a BUN test if he suspects kidney damage, notes Mayo Clinic. He may also order a BUN test as a part of a group of tests used to check for liver damage, gastrointestinal bleeding and other conditions.
When kidney function is impaired, BUN levels increase, according to the American Association for Clinical Chemistry. These levels also increase when an individual experiences obstructed urine flow or dehydration. A high-protein diet or excessive protein breakdown may also elevate BUN levels.