Answered by John Hsiao, M.D.:
Schizophrenia is diagnosed based on its symptoms, course of illness, and impairment. In the most recent Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR, American Psychiatric Association, Washington D.C., 2001), the diagnosis of schizophrenia requires a 1 month active period of symptoms (less if successfully treated) with two or more of the following: delusions, hallucinations, disorganized spech, disorganized or catatonic behavior, or negative symptoms (amotivation, lack of emotion, withdrawal). Including prodromal and residual periods, before and after development of active symptoms, the illness must have lasted at least six months, and be associated with significant social or occupational dysfunction. Finally, other causes of psychosis must be ruled out. In brief, schizophrenia is diagnosed when a person has severe symptoms of psychosis with marked impairment of function lasting 6 months or longer and not explainable by some other cause.