Your doctor may order a blood urea nitrogen test, also known as BUN test, to see how well your kidneys are working. A common blood test, the blood urea nitrogen (BUN) test reveals important information about how well your kidneys and liver are working. Urea is a waste product formed by the breakdown of protein and is cleared by the kidneys. A blood urea nitrogen (BUN) test is used to determine how well your kidneys are working. A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea. A BUN, or blood urea nitrogen test, can provide important information about your kidney function.