Blood Urea Nitrogen (BUN) is a blood test that measures the level of urea nitrogen in the blood. Urea is a waste product that is produced when the body breaks down protein. It is filtered out of the blood by the kidneys and excreted in the urine. An elevated level of BUN in the blood may indicate that the kidneys are not functioning properly, which can be caused by conditions such as dehydration, kidney disease, heart failure, or liver disease. In some cases, a high BUN level may also be due to certain medications or a high protein diet. Conversely, a low BUN level may indicate malnutrition, liver damage, or overhydration. The BUN test is often done as part of a comprehensive metabolic panel to evaluate kidney function and overall health.