Psychosis as a symptom of a psychiatric disorder is first and foremost a diagnosis of exclusion. So a new-onset episode of psychosis "cannot" be considered to be a symptom of a psychiatric disorder until other relevant and known medical causes of psychosis are excluded, or ruled out. Many clinicians improperly perform, or entirely miss this step, introducing avoidable diagnostic error and misdiagnosis.

An initial assessment includes a comprehensive history and physical examination. Although no biological laboratory tests exist which confirm schizoaffective disorder, biological tests should be performed to exclude psychosis associated with or caused by substance use, medications, toxins or poisons, surgical complications, or other medical illnesses. Since non-medical mental health practitioners are not trained to exclude medical causes of psychosis, people experiencing psychosis should be referred to an emergency department or hospital.

Delirium should be ruled out, which can be distinguished by visual hallucinations, acute onset and fluctuating level of consciousness, indicating other underlying factors which includes medical illnesses. Excluding medical illnesses associated with psychosis is performed by using blood tests to measure:

- Thyroid-stimulating hormone to exclude hypo- or hyperthyroidism,

- Basic electrolytes and serum calcium to rule out a metabolic disturbance,

- Full blood count including ESR to rule out a systemic infection or chronic disease, and

- Serology to exclude syphilis or HIV infection.

Other investigations which may be performed include:

- EEG to exclude epilepsy, and an

- MRI or CT scan of the head to exclude brain lesions.

Blood tests are not usually repeated for relapse in people with an established diagnosis of schizoaffective disorder, unless there is a specific "medical" indication. These may include serum BSL if olanzapine has previously been prescribed, thyroid function if lithium has previously been taken to rule out hypothyroidism, liver function tests if chlorpromazine has been prescribed, CPK levels to exclude neuroleptic malignant syndrome, and a urinalysis and serum toxicology screening if substance use is suspected. Assessment and treatment may be done on an outpatient basis; admission to an inpatient facility is considered if there is a risk to self or others.

Because psychosis may be precipitated or exacerbated by common classes of psychiatric medications, such as antidepressants, ADHD stimulant medications, and sleep medications, prescribed medication-induced psychosis should be ruled out, particularly for first-episode psychosis. This is an essential step to reduce diagnostic error and to evaluate potential medication sources of further patient harm. Regarding prescribed medication sources of patient harm, Yale School of Medicine Professor of Psychiatry Malcolm B. Bowers, Jr, MD wrote:

Illicit drugs aren't the only ones that precipitate psychosis or mania—prescribed drugs can too, and in particular, some psychiatric drugs. We investigated this and found that about 1 in 12 psychotic or manic patients in an inpatient psychiatric facility are there due to antidepressant-induced psychosis or mania. That's unfortunate for the field [of psychiatry] and disastrous for some of our patients.

Substance-induced psychosis should also be ruled out. Both substance- and medication-induced psychosis can be excluded to a high level of certainty while the person is psychotic, typically in an emergency department, using both a

- Broad spectrum urine toxicology screening, and a

- Full serum toxicology screening (of the blood).

Some dietary supplements may also induce psychosis or mania, but cannot be ruled out with laboratory tests. So a psychotic person's family, partner, or friends should be asked whether he or she is currently taking any dietary supplements.

Common mistakes made when diagnosing psychotic patients include:

- Not properly excluding delirium,

- Missing a toxic psychosis by not screening for substances "and" medications,

- Not appreciating medical abnormalities (e.g., vital signs),

- Not obtaining a medical history and family history,

- Indiscriminate screening without an organizing framework,

- Not asking family or others about dietary supplements,

- Premature diagnostic closure, and

- Not revisiting or questioning the initial diagnostic impression of primary psychiatric disorder.

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 person's 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.

Research efforts are focusing on prevention in identifying early signs from relatives with associated disorders similar with schizophrenia and those with prenatal and birth complications. Prevention has been an ongoing challenge because early signs of the disorder are similar to those of other disorders. Also, some of the schizophrenic related symptoms are often found in children without schizophrenia or any other diagnosable disorder.

The most widely used criteria for diagnosing schizoaffective disorder are from the American Psychiatric Association's "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 doesn't stay with most patients over time; and it has questionable diagnostic validity (that is, it doesn't 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 bipolar disorder or major depression). Only when psychotic states persist in a sustained fashion for two weeks or longer without concurrent affective symptoms is the diagnosis schizoaffective disorder or schizophrenia.

The second cardinal guideline in the DSM-5 diagnosis of schizoaffective disorder is one of timeframe.

These two changes are intended by the DSM-5 workgroup to accomplish two goals:

- Increase the diagnosis' consistency (or reliability) when it is used;

- Significantly decrease the overall use of the schizoaffective disorder diagnosis.

If the schizoaffective diagnosis is used less often, other diagnoses (like psychotic mood disorders and schizophrenia) are likely to be used more often; but this is hypothetical until real-world data arrive. Validity problems with the diagnosis remain and await further work in the fields of psychiatric genetics, neuroimaging, and cognitive science that includes the overlapping fields of cognitive, affective, and social neuroscience, which may change the way schizoaffective disorder is conceptualized and defined in future versions of the DSM and ICD.

The same criteria are used to diagnose children and adults. Diagnosis is based on reports by parents or caretakers, teachers, school officials, and others close to the child.

A professional who believes a child has schizophrenia usually conducts a series of tests to rule out other causes of behavior, and pinpoint a diagnosis. Three different types of exams are performed: physical, laboratory, and psychological. Physical exams usually cover the basic assessments, including but not limited to; height, weight, blood pressure, and checking all vital signs to make sure the child is healthy. Laboratory tests include electroencephalogram EEG screening and brain imaging scans. Blood tests are used to rule out alcohol or drug effects, and thyroid hormone levels are tested to rule out hyper- or hypothyroidism. A psychologist or psychiatrist talks to a child about their thoughts, feelings, and behavior patterns. They also inquire about the severity of the symptoms, and the effects they have on the child's daily life. They may also discuss thoughts of suicide or self-harm in these one-on-one sessions. Some symptoms that may be looked at are early language delays, early motor development delays and school problems.

Many of persons with childhood schizophrenia are initially misdiagnosed as having pervasive developmental disorders (autism spectrum disorder, for example).

This diagnostic rubric is not recommended for general use because it is not clearly demarcated either from simple schizophrenia or from schizoid or paranoid personality disorders, or possibly autism and Asperger syndrome as currently diagnosed. If the term is used, three or four of the typical features listed above should have been present, continuously or episodically, for at least 2 years. The individual must never have met criteria for schizophrenia itself. A history of schizophrenia in a first-degree relative gives additional weight to the diagnosis but is not a prerequisite.

Schizophreniform disorder is equally prevalent among men and women. The most common ages of onset are 18–24 for men and 18–35 for women. While the symptoms of schizophrenia often develop gradually over a period of years, the diagnostic criteria for schizophreniform disorder require a much more rapid onset.

Available evidence suggests variations in incidence across sociocultural settings. In the United States and other developed countries, the incidence is low, possibly fivefold less than that of schizophrenia. In developing countries, the incidence is substantially higher, especially for the subtype "With Good Prognostic Features". In some of these settings schizophreniform disorder may be as common as schizophrenia.

Although medication is the first-line treatment for most psychiatric disorders, it does not always improve every aspect of a patient's life, and for the negative symptoms in schizophrenia, the responses to anti-psychotics are less favourable than for positive symptoms. As a result, psychotherapy might be an alternative for the treatment of these symptoms, even if medication has a good effect on other manifestations of the disorder.

Cognitive behavioural therapy (CBT), is the kind of psychotherapy that shows most promise in treating avolition (and other negative symptoms of schizophrenia), but more research is needed in the area. CBT focuses on understanding how thoughts and feelings influence behaviour, in order to help individuals develop methods and strategies to better handle the implications of their disorder. Some research suggests that CBT focusing on social skills and practice of interpersonal situations, like job interviews, seeing a doctor (to discuss medication, for example), or interacting with friends and co-workers, as well as seemingly simple things like riding a bus, might reduce negative symptoms of schizophrenia and be beneficial to patients with avolition.

Other forms of psychotherapy might also complement the role of medication and help patients, their families, and friends to work through emotional and other challenges of living with a chronic psychological disorder, including avolition.

Various modalities of treatment, including pharmacotherapy, psychotherapy, and various other psychosocial and educational interventions, are used in the treatment of schizophreniform disorder. Pharmacotherapy is the most commonly used treatment modality as psychiatric medications can act quickly to both reduce the severity of symptoms and shorten their duration. The medications used are largely the same as those used to treat schizophrenia, with an atypical antipsychotic as the usual drug of choice. Patients who do not respond to the initial atypical antipsychotic may benefit from

being switched to another atypical antipsychotic, the addition of a mood stabilizer such as lithium or an anticonvulsant, or being switched to a typical antipsychotic.

Treatment of schizophreniform disorder can occur in inpatient, outpatient, and partial hospitalization settings. In selecting the treatment setting, the primary aims are to minimize the psychosocial consequences for the patient and maintain the safety of the patient and others. While the need to quickly stabilize the patient's symptoms almost always exists, consideration of the patient's severity of symptoms, family support, and perceived likelihood of compliance with outpatient treatment can help determine if stabilization can occur in the outpatient setting. Patients who receive inpatient treatment may benefit from a structured intermediate environment, such as a sub-acute unit, step-down unit, partial hospital, or day hospital, during the initial phases of returning to the community.

As improvement progresses during treatment, help with coping skills, problem-solving techniques, psychoeducational approaches, and eventually occupational therapy and vocational assessments are often very helpful for patients and their families. Virtually all types of individual psychotherapy are used in the treatment of schizophreniform disorder, except for insight-oriented therapies as patients often have limited insight as a symptom of their illness.

Since schizophreniform disorder has such rapid onset of severe symptoms, patients are sometimes in denial about their illness, which also would limit the efficacy of insight-oriented therapies. Supportive forms of psychotherapy such as interpersonal psychotherapy, supportive psychotherapy, and cognitive behavior therapy are particularly well suited for the treatment of the disorder. Group psychotherapy is usually not indicated for patients with schizophreniform disorder because they may be distressed by the symptoms of patients with more advanced psychotic disorders.

According to Theodore Millon, the schizotypal is one of the easiest personality disorders to identify but one of the most difficult to treat with psychotherapy. Persons with STPD usually consider themselves to be simply eccentric, productive, or nonconformist. As a rule, they underestimate maladaptiveness of their social isolation and perceptual distortions. It is not so easy to gain rapport with people who suffer from STPD due to the fact that increasing familiarity and intimacy usually increase their level of anxiety and discomfort. In most cases they do not respond to informality and humor.

Group therapy is recommended for persons with STPD only if the group is well structured and supportive. Otherwise, it could lead to loose and tangential ideation. Support is especially important for schizotypal patients with predominant paranoid symptoms, because they will have a lot of difficulties even in highly structured groups.

These are the current criteria:

The ICD is currently in revision and ICD-11 is expected to come out in 2018. In the preliminary Beta Draft version, there is no longer a diagnostic category of simple schizophrenia and all subtypes have been eliminated.

Implications from avolition often result in social deficits. Not being able to initiate and perform purposeful activities can have many implications for a person with avolition. By disrupting interactions with both familiar and unfamiliar people, it jeopardizes the patient's social relations. When part of a severe mental illness, avolition has been reported, in first person accounts, to lead to physical and mental inability to both initiate and maintain relationships, as well as work, eat, drink or even sleep.

Clinically, it may be difficult to engage an individual experiencing avolition in active participation of psychotherapy. Patients are also faced with the stresses of coping with and accepting a mental illness and the stigma that often accompanies such a diagnosis and its symptoms. Regarding schizophrenia, the American Psychiatric Association reported in 2013 that there currently are "no treatments with proven efficacy for primary negative symptoms" (such as avolition). Together with schizophrenia's chronic nature, such facts added to the outlook of never getting well, might further implicate feelings of hopelessness and similar in patients as well as their friends and family.

This form of schizophrenia is typically associated with early onset (often between the ages of 15 and 25 years) and is thought to have a poor prognosis because of the rapid development of negative symptoms and decline in social functioning.

Use of electroconvulsive therapy has been proposed; however, the effectiveness after treatment is in question.

The use of antipsychotic medication is commonly the first line of treatment; however, the effectiveness after treatment is in question.

L-DOPA is effective against reduced affect display and emotional withdrawal, aloofness from society, apathy.

Reduced affect display, sometimes referred to as emotional blunting, is a condition of reduced emotional reactivity in an individual. It manifests as a failure to express feelings (affect display) either verbally or non-verbally, especially when talking about issues that would normally be expected to engage the emotions. Expressive gestures are rare and there is little animation in facial expression or vocal inflection. Reduced affect can be symptomatic of autism, schizophrenia, depression, posttraumatic stress disorder, depersonalization disorder, schizoid personality disorder or brain damage. It may also be a side effect of certain medications (e.g., antipsychotics and antidepressants). Individuals with blunted or flat affect show different regional brain activity when compared with typical individuals.

Reduced affect should be distinguished from apathy, which explicitly refers to a lack of emotion, whereas reduced affect is a lack of emotional expression regardless of whether emotion is actually reduced or not.

Disorganized schizophrenia, also known as hebephrenia or hebephrenic schizophrenia, is a subtype of schizophrenia, although it is not recognized in the latest version of the "Diagnostic and Statistical Manual of Mental Disorders". It's recognized only in the 10th revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10).

Disorganized schizophrenia is thought to be an extreme expression of the "disorganization syndrome" that has been hypothesized to be one aspect of a three-factor model of symptoms in schizophrenia, the other factors being "reality distortion" (involving delusions and hallucinations) and "psychomotor poverty" (lack of speech, lack of spontaneous movement and various aspects of blunting of emotion).

A restricted or constricted affect is a reduction in an individual's expressive range and the intensity of emotional responses.

Multiple complex developmental disorder (MCDD) is a research category, proposed to involve several neurological and psychological symptoms where at least some symptoms are first noticed during early childhood and persist throughout life. It was originally suggested to be a subtype of autistic spectrum disorders (PDD) with co-morbid schizophrenia or another psychotic disorder; however, there is some controversy that not everyone with MCDD meets criteria for both PDD and psychosis. The term "multiplex developmental disorder" was coined by Donald J. Cohen in 1986.

Multiple complex developmental disorder is likely to be caused by a number of different various genetic factors. Each individual with MCDD is unique from one another and displays different symptoms. Various neuropsychological disorders can also be found in family members of people with MCDD.