BUN measures the amount of urea in a sample of blood. Urea is a waste product that forms as part of the body’s natural process of breaking down proteins. It is also referred to as urea nitrogen and is filtered out of the blood by the kidneys. A BUN test is most often interpreted together with creatinine to help assess how well the kidneys are working. While BUN can be tested independently, it is normally included in a panel with other measurements, which offers a more extensive evaluation of the condition of your body.