DefinitionBy Mayo Clinic staff
A blood urea nitrogen (BUN) test reveals important information about how well your kidneys and liver are working.
A blood urea nitrogen test measures the amount of urea nitrogen that's in your blood. 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 other waste products from your blood. The filtered waste products leave your body in urine.
If a blood urea nitrogen test reveals that your urea nitrogen levels are higher than normal, it probably indicates that your kidneys aren't working properly. Or it could point to high protein intake, inadequate fluid intake or poor circulation.
If a blood urea nitrogen test shows lower than normal levels, it could indicate liver disease or damage, or malnutrition. But a low BUN level wouldn't likely be the first indication of liver disease because the blood urea nitrogen test isn't used as a screening test for that disorder.
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