Abstract: A method of detecting inflammatory processes comprises (a) administering per-orally or per-rectally a composition comprising a diagnostically effective amount of L-[guanido-15N2]-arginine, L-[guanido-15N]-arginine, their mixtures and pharmaceutically acceptable salts thereof, and a pharmaceutically acceptable carrier; (b) collecting a sample of exhaled air, saliva or urine; (c) determining 13NO in the air sample or its transformation products 15IN-nitrite and/or 15N-nitrate in the saliva sample or urine. Also disclosed is a corresponding diagnostic composition for use in diagnosing inflammatory processes and a method for its manufacture.