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.
Current studies employing the use of FDOPA PET scanning (FDOPA PET) as a possible method for identifying CBD have focused on analyzing the efficiency of neurons in the striatum that utilize the neurotransmitter dopamine. These studies have concluded that, in general, dopamine uptake was diminished in the caudate and the putamen. This characteristic also has the potential to be useful in distinguishing CBD from the similar PD, as individuals having been diagnosed with PD were more likely to have a lower uptake of dopamine than in individuals with CBD.Other clinical tests or procedures that monitor the presence of dopamine within the brain (β-CIT SPECT and IBZM SPECT) have shown similar findings. β-CIT serves as an indicator for presynaptic dopaminergic neurons, whereas IBZM is a tracer that shows an affinity for the postsynaptic neurons of the same type. Despite agreement with other imaging studies, these two SPECT methods suffer some scrutiny due to better accuracy in other imaging methods. However, β-CIT SPECT has proven to be helpful in distinguishing CBD from PSP and multiple system atrophy (MSA). Corticobasal syndrome
All of the disorders and dysfunctions associated with CBD can often be categorized into a class of symptoms that present with the disease of CBD. These symptoms that aid in clinical diagnosis are collectively referred to as corticobasal syndrome (CBS) or corticobasal degeneration syndrome (CBDS). Alzheimers disease, Picks disease, FTDP-17 and progressive supranuclear palsy can display a corticobasal syndrome.