BUN test or blood urea nitrogen test is done to measure the amount of nitrogen in urea in order to know kidney function. Its normal levels are 7-20 mg/dl. Higher levels can be due to kidney diseases, kidney failure, shock, urinary tract obstruction, heart attack, congestive heart failure, hypovolemia, excessive protein levels and gastrointestinal bleeding. Below normal levels can be due to over hydration, liver failure, malnutrition and low protein diet. Your doctor can diagnose your exact problem after co-relating with clinical symptoms.