A urea test (sometimes called a blood urea nitrogen, or BUN test) measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is formed when protein is broken down in your body. It is made in the liver and removed from your body in urine.

A urea test is done to tell how well your kidneys are working. If your kidneys are not able to remove urea from the blood normally, your urea level increases. Heart failure, dehydration, certain medicines, intestinal bleeding, or a diet high in protein also can increase your urea level. Liver disease or damage can decrease your urea level, because urea is made in the liver. A decreased urea level can occur normally in the second or third trimester of pregnancy.