what does bun mean
Blood urea nitrogen (BUN) test. Overview. A common blood test, the blood urea nitrogen (BUN) test reveals important information about how well your kidneys and liver are working. A BUN test measures the amount of urea nitrogen that's in your blood. 1  Your liver produces ammonia â which contains nitrogen â after it breaks down proteins used by your body's cells.  The nitrogen combines with other elements, such as carbon, hydrogen and oxygen, to form urea, which is a chemical waste product.