Blood Urea Nitrogen (BUN) test measures the amount of nitrogen in the blood that comes from the waste product urea, which is made when protein is broken down. Urea is made in the liver and passed out of the body in urine. The BUN test is primarily used, along with the creatinine test, to evaluate kidney function in a wide range of circumstances, to help diagnose kidney disease, and to monitor patients with acute or chronic kidney dysfunction or failure. This test is also used to monitor the effectiveness of dialysis and other treatments related to kidney disease or damage. HIGH BUN levels are associated with impaired kidney function due to acute or chronic kidney disease, damage, or failure. It may also be due to a condition that results in decreased blood flow to the kidneys, such as congestive heart failure, shock, stress, recent heart attack, or severe burns, to conditions that cause obstruction of urine flow, or to dehydration. LOW levels are not common but may be seen in severe liver disease, malnutrition, and sometimes when a patient is over-hydrated.