Diagnostic method, based on the amounts of biogenic amines that are contained in a body fluid or other sample. A number of measured parameters related to the desired diagnostic information are derived from the amounts. For each diagnostic information desired, an input consisting of the identification of the diagnostic information is provided. The input is compared to the measured parameters and a diagnostic response is derived from the comparison. The measured parameters may be derived from the amounts of the biogenic amines according to a program stored in a memory. The detection of the less volatile amines and their separation from the more volatile ones may advantageously be enhanced by successively adding a base and an acid, in either possible succession, to the sample and analyzing the vapors emitted by the sample under heating.