Blood Urea Nitrogen (BUN) is a medical test that measures the amount of urea nitrogen in the blood. Urea is a waste product formed when the body breaks down proteins. The kidneys filter urea from the blood, and its levels can indicate how well the kidneys are functioning. A high BUN level may suggest kidney dysfunction, dehydration, or excessive protein intake, while a low level can indicate liver disease or malnutrition. Doctors often use BUN results alongside other tests to assess a patient's overall health and kidney performance.