BUN, or Blood Urea Nitrogen, is a medical test that measures the amount of urea nitrogen in the blood. Urea is a waste product formed in the liver when the body breaks down proteins. The kidneys filter urea from the blood, and high levels of BUN can indicate that the kidneys are not functioning properly or that there is an issue with protein metabolism. The BUN test is often used alongside other tests, such as creatinine, to assess kidney health. Normal BUN levels typically range from 7 to 20 mg/dL, but these values can vary based on factors like age, diet, and hydration status.