The Collaborative International Dictionary of English v.0.48:

Diagnostics \Di`ag*nos"tics\, n.
   That part of medicine which has to do with ascertaining the
   nature of diseases by means of their symptoms or signs.
   [1913 Webster]

         His rare skill in diagnostics.           --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diagnostics
    n 1: the branch of medical science dealing with the
         classification of disease [syn: nosology, diagnostics]