In some ways, the diagnosis of schizophrenia should be considered a diagnosis of exclusion due to both the nonspecificity of psychotic symptoms and the severity of the consequences associated with the diagnosis. A diagnosis of schizophrenia must be based on the presence of specific symptoms and a specific course of symptoms as defined in DSM-IV. Other psychiatric disorders can generally be differentiated from schizophrenia if symptoms are narrowly defined and the course of illness is carefully taken into account. It is important to obtain a thorough history and medical evaluation (i.e., physical exam, general chemistry screen, complete blood count, urinalysis, and drug screen) to rule out organic causes (i.e., medical and neurological conditions or substance abuse) for the psychotic symptoms. Atypical presentations, such as abrupt onset of symptoms, clouding of sensorium, or onset after age 30 should be carefully investigated. When the history and psychiatric examination of a psychotic patient do not reveal a pattern of symptoms and symptom duration that clearly corresponds to a diagnosis of schizophrenia or another well-defined disorder, it is best to use the category of psychosis NOS until more information can be obtained.
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