Urea nitrogen is a waste product formed when protein breaks down, states MedlinePlus. Urea nitrogen is measured by taking a blood sample, and the result is reported as BUN, or blood urea nitrogen. Depending on the lab used, a normal BUN generally ranges from 6 to 20 milligrams per deciliter.
The main purpose of the BUN test is to check kidney function, according to MedlinePlus. Elevated BUN levels sometimes indicate congestive heart failure, heart attack, dehydration or bleeding in the digestive tract. Kidney problems, urinary tract obstruction and shock can also lead to BUN levels that are higher than normal. Low BUN levels are usually a result of a low protein diet, malnutrition, over-hydration or liver failure.