Objective: To review the literature regarding the diagnosis of first-episode psychosis in the context of psychiatric emergency. Method: Review of empirical and review articles selected by electronic search in the PubMed database. Results: Specific features of emergency care -single, brief, and cross-sectional assessments with little information -may jeopardize the diagnostic process. These limitations can be circumvented by the application of operational diagnostic criteria, by the use of scales and structured interviews, and by short observation periods (24-72 hours). Diagnoses of bipolar disorder, schizophrenia, psychotic depression, and delusional disorder performed in the context of emergency have good stability, but the same does not apply to the diagnoses of brief psychotic disorder, schizophreniform disorder, and schizoaffective disorder. Firstepisode psychosis can occur in the course of the use of psychoactive substances and the persistence of psychotic symptoms even after cessation of the use of the substance is relatively frequent. The rational use of subsidiary tests may be of help in establishing the differential diagnosis of psychotic episodes due to general medical conditions. Conclusion: The diagnosis of first-episode psychosis can be adequately performed by psychiatric emergency services if evidence-based routines are implemented.
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