There are several theorized causations for the onset of Schizoaffective disorder, including, genetics, general brain function, like chemistry, and structure, and stress.Only after these relevant and known causes of psychosis have been ruled out can a psychiatric differential diagnosis be made. A mental health clinician will incorporate family history, observation of a psychotic persons behavior while the person is experiencing active symptoms, to begin a psychiatric differential diagnosis. Diagnosis also includes self-reported experiences, as well as behavioral abnormalities reported by family members, friends, or significant others. Mistakes in this stage include:

Not screening for dissociative disorders. Dissociative identity disorder and psychotic symptoms in schizoaffective disorder have considerable overlap, yet a different overall treatment approach. DSM-5 criteria
The most widely used criteria for diagnosing schizoaffective disorder are from the American Psychiatric Associations Diagnostic and Statistical Manual of Mental Disorders-5.The DSM-IV schizoaffective disorder definition was plagued by problems of being inconsistently (or unreliably) used on patients; when the diagnosis is made, it does not stay with most patients over time, and it has questionable diagnostic validity (that is, it does not describe a distinct disorder, nor predict any particular outcome). These problems have been slightly reduced (or "modestly improved") in the DSM-5 according to Carpenter.When psychotic symptoms are confined to an episode of mania or depression (with or without mixed features), the diagnosis is that of a “psychotic” mood disorder, namely either psychotic bipolar disorder or psychotic major depression.
The categorical diagnostic manuals do not reflect reality in their separation of psychosis (via the schizophrenia diagnosis) from mood disorder, nor do they currently emphasize the actual overlap found in real-life patients. Thus, they are likely to continue to introduce either-or conceptual and diagnostic error, by way of confirmation bias into clinicians mindsets, hindering accurate assessment and treatment.The new definition continues the lack of parsimony of the old definition. Simpler, clearer, and more usable definitions of the diagnosis were supported by certain members of the DSM-5 workgroup (see next paragraph); these were debated but deemed premature, because more "research [is] needed to establish a new classification system of equal or greater validity" to the existing system. Because of DSM-5s continuing problematic categorical foundation, schizoaffective disorders conceptual and diagnostic validity remains doubtful. After enough research is completed and data exists, future diagnostic advances will need to either eliminate and replace, or soften and bridge, the hard categorical separation of mood disorders from schizophrenia; most likely using a spectrum or dimensional approach to diagnosis.More parsimonious definitions than the current one were considered by Carpenter and the DSM-5 workgroup:
One option for the DSM-5 would have been to remove the schizoaffective disorder category and to add affective [or mood] symptoms [that is, mania, hypomania, mixed episode, or depression] as a dimension to schizophrenia and schizophreniform disorder or to define a single category for the co-occurrence of psychosis and mood symptoms.