Blood urea nitrogen (BUN) is a medical test that measures the amount of urea nitrogen found in blood. The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain 6 to 20 mg/dL (2.1 to 7.1 mmol/L) of urea nitrogen. Individual laboratories will have different reference ranges as the assay used can vary between laboratories. The test is used to detect renal problems. It is not considered as reliable as creatinine or BUN/creatinine ratio blood studies.