Normal blood urea nitrogen levels generally range from 7 to 20 milligrams per deciliter, according to Mayo Clinic. This range, however, varies with age and the reference range that the laboratory uses. An individual needs to seek an explanation of his BUN test results from the doctor.
When the age of an individual increases, his urea nitrogen level also increases, explains Mayo Clinic. An elevated blood urea nitrogen level occurs when the kidneys malfunction. Other factors that result in high blood urea nitrogen levels include congestive heart failure, gastrointestinal bleeding, urinary tract obstruction and dehydration. Eating high-protein foods also cause the blood urea nitrogen level to rise.