A blood urea nitrogen (BUN) test measures the amount of urea nitrogen found in your blood. Urea nitrogen is a waste product made when your liver breaks down protein. It's carried in your blood, filtered out by your kidneys, and removed from your body in your urine. If your liver isn't healthy, it may not break down proteins the way it should. And if your kidneys aren't healthy, they may not properly filter urea. Either of these problems can lead to larger amounts of urea nitrogen in your body. BUN levels help your healthcare provider see how well your kidneys are working. The test may be used along with other measurements to help diagnose a kidney disorder or find out how well your treatment for kidney disease is working.