nonprotein nitrogen (NPN) /-prō′tēn/ [L, non + Gk, proteios, first rank, nitron, soda, genein, to produce] , the nitrogen in the blood that is not a constituent of protein, such as the nitrogen associated with urea, uric acid, creatine, and polypeptides. Approximately one half of the nonprotein nitrogen in the blood is associated with urea.
