Blood urea nitrogen
![](/Images/godic/202502/10/Urea-3D-balls5430.png")
![Reference ranges for blood tests, comparing urea (yellow at right) to other blood constituents.](/Images/godic/202502/10/Blood_values_sorted_by_mass_and_molar_concentration5430.png")
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.