A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine. blood urea nitrogen (BUN) test is done to: 1  See if your kidneys are working normally. 2  See if your kidney disease is getting worse. 3  See if treatment of your kidney disease is working. 4  Check for severe dehydration.