Blood urea nitrogen, usually referred to by the abbreviation BUN in medicine, is a measure of the amount of urea in the blood. Like creatinine, it is used as a gauge of the health of the kidneys, which normally remove excess urea from the blood. Depending on the testing lab (which use different reference ranges due to different testing procedures), a result of anywhere from 7-21 milligrams of urea per 100 millilitres of blood is normal.
Although kidney failure will cause high BUN levels, a differential diagnosis must still consider such factors as lost blood volume, fever, metabolic conditions that speed up the breakdown of large molecules in the body, a diet high in protein and disorders of the digestive tract.