blood urea nitrogen (bun) is a metabolic by product (in the liver) from the breakdown of blood, muscle and protein. Blood urea nitrogen can be measured from a simple venipuncture specimen. Abnormal elevation in the blood urea nitrogen can indicate renal disease, dehydration, congestive heart failure, gastrointestinal bleeding, starvation, shock or urinary tract obstruction (by tumour or prostate gland). Low bun level can indicate liver disease, malnutrition or a low protein diet. Normal bun levels should be between 7 and 20 mg/dl (milligrams per decilitre).