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.Here's how your body typically forms and gets rid of urea nitrogen:Your liver produces ammonia; which contains nitrogen; 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.The urea travels from your liver to your kidneys through your bloodstream.Healthy kidneys filter urea and remove other waste products from your blood.The filtered waste products leave your body through urine.A BUN test can reveal whether your urea nitrogen levels are higher than normal, suggesting that your kidneys or liver may not be working properly.