diagnostic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Of, relating to, or used in a diagnosis.
• adjective Serving to identify a particular disease; characteristic.
• noun The art or practice of medical diagnosis.
• noun A symptom or a distinguishing feature serving as supporting evidence in a diagnosis.
• noun An instrument or technique used in medical diagnosis.

from The Century Dictionary.

• Of or pertaining to diagnosis; determining a diagnosis; indicating the nature; constituting a ground of discrimination.
• noun In pathology, a symptom of value in diagnosis.
• noun In zoology and botany, a term or phrase which constitutes a diagnosis; a definition or characterization.

from the GNU version of the Collaborative International Dictionary of English.

• noun The mark or symptom by which one disease is known or distinguished from others.
• adjective Pertaining to, or furnishing, a diagnosis; indicating the nature of a disease.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective of, or relating to diagnosis
• adjective characteristic of a particular disease
• noun a technique etc. used in medical diagnosis
• noun computing Any tool or technique used to find the root of a problem

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective characteristic or indicative of a disease
• adjective concerned with diagnosis; used for furthering diagnosis