urea is not protein.  It fools a protein test because all a protein test is - is a measure of nitrogen and assumes you are testing something cellular and deduces how much protein is present.  the protein content of urea is 0%

on a side note, the chinese thought it economically advantageous to water down baby milk formula by using less milk and mix in water and melamine.  protein "tested" the same.  outcome was bad.