A diagnosis is the determination by a medical professional of the nature of a person’s disorder after consideration of their signs, symptoms and the results of medical tests. A diagnosis categorises a person’s illness into a disorder that is characterised by the symptoms they are experiencing.

For example, delusions and hallucinations experienced by a person are characteristic of an amphetamine overdose, or of schizophrenia. If drug use can be ruled out and the other criteria for a diagnosis is met, the person will be diagnosed with schizophrenia.