Differential diagnosis

To make a diagnosis of schizophrenia other possible causes of psychosis need to be excluded.  Psychotic symptoms lasting less than a month may be diagnosed as brief psychotic disorder, or as schizophreniform disorder. Psychosis is noted in Other specified schizophrenia spectrum and other psychotic disorders as a DSM-5 category. Schizoaffective disorder is diagnosed if symptoms of mood disorder are substantially present alongside psychotic symptoms. Psychosis that results from a general medical condition or substance is termed secondary psychosis.

Psychotic symptoms may be present in several other conditions, including bipolar disorder, borderline personality disorder, substance intoxication, substance-induced psychosis, and a number of drug withdrawal syndromes. Non-bizarre delusions are also present in delusional disorder, and social withdrawal in social anxiety disorder, avoidant personality disorder and schizotypal personality disorder. Schizotypal personality disorder has symptoms that are similar but less severe than those of schizophrenia. Schizophrenia occurs along with obsessive–compulsive disorder (OCD) considerably more often than could be explained by chance, although it can be difficult to distinguish obsessions that occur in OCD from the delusions of schizophrenia. There can be considerable overlap with the symptoms of post-traumatic stress disorder.

A more general medical and neurological examination may be needed to rule out medical illnesses which may rarely produce psychotic schizophrenia-like symptoms, such as metabolic disturbance, systemic infection, syphilis, HIV-associated neurocognitive disorder, epilepsy, limbic encephalitis, and brain lesions. Stroke, multiple sclerosis, hyperthyroidism, hypothyroidism, and dementias such as Alzheimer's disease, Huntington's disease, frontotemporal dementia, and the Lewy body dementias may also be associated with schizophrenia-like psychotic symptoms. It may be necessary to rule out a delirium, which can be distinguished by visual hallucinations, acute onset and fluctuating level of consciousness, and indicates an underlying medical illness. Investigations are not generally repeated for relapse unless there is a specific medical indication or possible adverse effects from antipsychotic medication. In children hallucinations must be separated from typical childhood fantasies. It is difficult to distinguish childhood schizophrenia from autism.