Differential diagnosis and diagnostic systems in schizophrenia.

Diagnosis and prognosis are critical issues confronting psychiatry. In order to answer the fundamental questions concerning the origin and development of schizophrenia, we must first be clear about what it is. We must be able to separate the illness of schizophrenia from other disorders (see Fig. 1). We have attempted in this article to examine some of the illnesses that may resemble schizophrenia and make its discrimination difficult. We hope that by discussing these disorders and their similarities to schizophrenia the important issues and dilemmas have become clearer and more readily understood by the clinician. Future studies assessing the validity of diagnostic systems for schizophrenia may have to rely on features other than cross-sectional symptoms and longitudinal course. Such characteristics as pharmacologic responsivity and genetic transmission and the development of biologic markers may be the prospective cornerstones for validating the diagnosis of schizophrenia.