Given the task definition and input, reply with output. Given a text passage, come up with an appropriate title for it. The title should accurately represent what the main focus of the passage is. The title of a passage may be present in the first few sentences of the passage and/or be referred to multiple times in the passage. The title should be 1-5 words long.

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 between 6 and 20 mg of urea nitrogen per 100 ml ( 6 -- 20 mg / dL ) of blood . Individual laboratories will have different reference ranges as the assay used will vary between laboratories .
Blood urea nitrogen