The person's appearance, behavior, and history, along with a psychological evaluation, are usually sufficient to establish a diagnosis. There is no test to confirm this diagnosis. Because the criteria are subjective, some people may be wrongly diagnosed.

The previous edition of the Diagnostic and Statistical Manual of Mental Disorders, DSM IV-TR, defines histrionic personality disorder (in Cluster B) as:

The DSM-IV requires that a diagnosis for any specific personality disorder also satisfies a set of general personality disorder criteria.

In November 2016, the American Association of Sexuality Educators, Counselors and Therapists (AASECT), the official body for sex and relationship therapy in the United States, issued a position statement on Sex Addiction which states that AASECT "does not find sufficient empirical evidence to support the classification of sex addiction or porn addiction as a mental health disorder, and does not find the sexual addiction training and treatment methods and educational pedagogies to be adequately informed by accurate human sexuality knowledge. Therefore, it is the position of AASECT that linking problems related to sexual urges, thoughts or behaviors to a porn/sexual addiction process cannot be advanced by AASECT as a standard of practice for sexuality education delivery, counseling or therapy."

In 2017, three new USA sexual health organizations found no support for the idea that sex or adult films were addictive in their position statement.

In November 16, 2017 the Association for the Treatment of Sexual Abusers (ATSA) published a position against sending sex offenders to sex addiction treatment facilities. Those centers argued that "illegal" behaviors were symptoms of sex addiction, which ATSA challenged they had no scientific evidence to support.

Because of reduced levels of trust, there can be challenges in treating PPD. However, psychotherapy, antidepressants, antipsychotics and anti-anxiety medications can play a role when an individual is receptive to intervention.

Accurately assessing for a specific Depressive Disorder diagnosis requires an expenditure of time that is deemed unreasonable for most primary care physicians. For this reason, physicians often use this code as a proxy for a more thorough diagnosis. There is concern that this may lead to a "wastebasket" mindset for certain disorders. In addition reimbursement through Medicare may be lower for certain non specific diagnosis.

About 25-40% of youths diagnosed with conduct disorder qualify for a diagnosis of antisocial personality disorder when they reach adulthood. For those that do not develop ASPD, most still exhibit social dysfunction in adult life.

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.

Conduct disorder is classified in the fourth edition of "Diagnostic and Statistical Manual of Mental Disorders" (DSM). It is diagnosed based on a prolonged pattern of antisocial behaviour such as serious violation of laws and social norms and rules in people younger than the age of 18. Similar criteria are used in those over the age of 18 for the diagnosis of antisocial personality disorder. No proposed revisions for the main criteria of conduct disorder exist in the "DSM-5"; there is a recommendation by the work group to add an additional specifier for callous and unemotional traits. According to DSM-5 criteria for conduct disorder, there are four categories that could be present in the child's behavior: aggression to people and animals, destruction of property, deceitfulness or theft, and serious violation of rules.

Almost all adolescents who have a substance use disorder have conduct disorder-like traits, but after successful treatment of the substance use disorder, about half of these adolescents no longer display conduct disorder-like symptoms. Therefore, it is important to exclude a substance-induced cause and instead address the substance use disorder prior to making a psychiatric diagnosis of conduct disorder.

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.

Diagnosis is based on the self-reported experiences of the person followed by a clinical assessment. Psychiatric assessment includes a psychiatric history and some form of mental status examination. Since some medical and psychiatric conditions mimic the symptoms of DPD, clinicians must differentiate between and rule out the following to establish a precise diagnosis: temporal lobe epilepsy, panic disorder, acute stress disorder, schizophrenia, migraine, drug use, brain tumour or lesion. No laboratory test for depersonalization-derealization disorder currently exists.

The diagnosis of depersonalization disorder can be made with the use of the following interviews and scales:

The Structured Clinical Interview for DSM-IV Dissociative Disorders (SCID-D) is widely used, especially in research settings. This interview takes about 30 minutes to 1.5 hours, depending on individual's experiences.

The Dissociative Experiences Scale (DES) is a simple, quick, self-administered questionnaire that has been widely used to measure dissociative symptoms. It has been used in hundreds of dissociative studies, and can detect depersonalization and derealization experiences.

The Dissociative Disorders Interview Schedule (DDIS) is a highly structured interview which makes DSM-IV diagnoses of somatization disorder, borderline personality disorder and major depressive disorder, as well as all the dissociative disorders. It inquires about positive symptoms of schizophrenia, secondary features of dissociative identity disorder, extrasensory experiences, substance abuse and other items relevant to the dissociative disorders. The DDIS can usually be administered in 30–45 minutes.

The Cambridge Depersonalization Scale (CDS) is a method for determining the severity of depersonalization disorder. It has been proven and accepted as a valid tool for the diagnosis of depersonalization disorder in a clinical setting. It is also used in a clinical setting to differentiate minor episodes of depersonalization from actual symptoms of the disorder. Due to the success of the CDS, a group of Japanese researchers underwent the effort to translate the CDS into the J-CDS or the Japanese Cambridge Depersonalization Scale. Through clinical trials the Japanese research team successfully tested their scale and determined its accuracy. One limitation is that the scale does not allow for the differentiation between past and present episodes of depersonalization. It should also be noted that it may be difficult for the individual to describe the duration of a depersonalization episode, and thus the scale may lack accuracy. The project was conducted in the hope that it would stimulate further scientific investigations into depersonalization disorder.

There is a considerable personality disorder diagnostic co-occurrence. Patients who meet the DSM-IV-TR diagnostic criteria for one personality disorder are likely to meet the diagnostic criteria for another. Diagnostic categories provide clear, vivid descriptions of discrete personality types but the personality structure of actual patients might be more accurately described by a constellation of maladaptive personality traits.

"Sites used DSM-III-R criterion sets. Data obtained for purposes of informing the development of the DSM-IV-TR personality disorder diagnostic criteria."

Abbreviations used: "PPD – Paranoid Personality Disorder, SzPD – Schizoid Personality Disorder, StPD – Schizotypal Personality Disorder, ASPD – Antisocial Personality Disorder, BPD – Borderline Personality Disorder, HPD – Histrionic Personality Disorder, NPD – Narcissistic Personality Disorder, AvPD – Avoidant Personality Disorder, DPD – Dependent Personality Disorder, OCPD – Obsessive-Compulsive Personality Disorder, PAPD – Passive-Aggressive Personality Disorder."

Some mental health providers have proposed various, but similar, criteria for diagnosing sexual addiction, including [[Patrick Carnes]], and [[Aviel Goodman]]. Carnes authored the first clinical book about sex addiction in 1983, based on his own empirical research. His diagnostic model is still largely utilized by the thousands of certified sex addiction therapists (CSATs) trained by the organization he founded. No diagnostic proposal for sex addiction has been adopted into any official government diagnostic manual, however.

During the update of the Diagnostic and Statistical Manual to version 5 (DSM-5), the APA rejected two independent proposals for inclusion.

In 2011, the [[American Society of Addiction Medicine]] (ASAM), the largest medical consensus of physicians dedicated to treating and preventing addiction, redefined addiction as a chronic brain disorder, which for the first time broadened the definition of addiction from substances to include addictive behaviors and reward-seeking, such as gambling and sex.

Depersonalization disorder is classified differently in the DSM-IV-TR and in the ICD-10: In the DSM-IV-TR this disorder it is seen as a dissociative disorder; in the ICD-10 as an independent neurotic disorder. Whether depersonalization disorder should be characterized as a dissociative disorder can be discussed.

Dependent personality disorder occurs in about 0.6% of the general population. The disorder is diagnosed more often in females than males; however, research suggests that this is largely due to behavioural differences in interviews and self-reporting rather than a difference in prevalence between the sexes. A 2004 twin study suggests a heritability of 0.81 for developing dependent personality disorder. Because of this, there is significant evidence that this disorder runs in families. Children and adolescents with a history of anxiety disorders and physical illnesses are more susceptible to acquiring this disorder.

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.

People with hypersexual disorder experience multiple, unsuccessful attempts to control or diminish the amount of time spent engaging in sexual fantasies, urges, and behaviors in response to dysphoric mood states or stressful life events.

For a valid diagnosis of hypersexual disorder to be established, symptoms must persist for a period of at least 6 months and occur independently of a use mania or a medical condition.

Psychiatrist Kantor suggests a treatment approach using psychodynamic, supportive, cognitive, behavioral and interpersonal therapeutic methods. These methods apply to both the Passive–aggressive person and their target victim.

Currently, genetic research for the understanding of the development of personality disorders is severely lacking. However, there are a few possible risk factors currently in discovery. Researchers are currently looking into genetic mechanisms for traits such as aggression, fear and anxiety, which are associated with diagnosed individuals. More research is being conducted into disorder specific mechanisms.

The World Health Organization's ICD-10 lists dependent personality disorder as " Dependent personality disorder":

It is characterized by at least 4 of the following:

1. Encouraging or allowing others to make most of one's important life decisions;

2. Subordination of one's own needs to those of others on whom one is dependent, and undue compliance with their wishes;

3. Unwillingness to make even reasonable demands on the people one depends on;

4. Feeling uncomfortable or helpless when alone, because of exaggerated fears of inability to care for oneself;

5. Preoccupation with fears of being abandoned by a person with whom one has a close relationship, and of being left to care for oneself;

6. Limited capacity to make everyday decisions without an excessive amount of advice and reassurance from others.

Associated features may include perceiving oneself as helpless, incompetent, and lacking stamina.

Includes:

- Asthenic, inadequate, passive, and self-defeating personality (disorder)

It is a requirement of ICD-10 that a diagnosis of any specific personality disorder also satisfies a set of general personality disorder criteria.

Despite recent initiatives to study psychopathology along dimensions of behavior and neurobiological indices, which would help refine a clearer picture of the development and treatment of externalizing disorders, the majority of research has examined specific mental disorders. Thus, best practices for many externalizing disorders are disorder-specific. For example, substance use disorders themselves are very heterogeneous and their best-evidenced treatment typically includes cognitive behavioral therapy, motivational interviewing, and a substance disorder-specific detoxification or psychotropic medication treatment component. The best-evidenced treatment for childhood conduct and externalizing problems more broadly, including youth with ADHD, ODD, and CD, is parent management training, a form of cognitive behavioral therapy. Additionally, individuals with ADHD, both youth and adults, are frequently treated with stimulant medications (or alternative psychotropic medications), especially if psychotherapy alone has not been effective in managing symptoms and impairment. Psychotherapy and medication interventions for individuals with severe, adult forms of antisocial behavior, such as antisocial personality disorder, have been mostly ineffective. An individual's comorbid psychopathology may also influences the course of treatment for an individual.

Once the patient and family have been educated about the nature, management and treatment of the disorder and a decision has been made to treat, the European ADHD Guidelines group recommends medication rather than behavioral training as the first treatment approach; and the UK's National Institute for Health and Clinical Excellence recommends medication as first line treatment for those with hyperkinesis/severe ADHD, and the provision of group parent-training in all cases of ADHD.

Data from the 2001–02 National Epidemiologic Survey on Alcohol and Related Conditions indicates a prevalence rate of 2.36% in the American general population. It appears to occur with equal frequency in males and females. In one study, it was seen in 14.7% of psychiatric outpatients.

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.

Bipolar disorder is difficult to diagnose. If a person displays some symptoms of bipolar disorder but not others, the clinician may diagnose bipolar NOS. The diagnosis of bipolar NOS is indicated when there is a rapid change (days) between manic and depressive symptoms and can also include recurring episodes of hypomania. Bipolar NOS may be diagnosed when it is difficult to tell whether bipolar is the primary disorder due to another general medical condition, such as a substance use disorder.

ADHD often precedes the onset of ODD, and approximately half of children with ADHD, Combined Type also have ODD. ODD is a risk factor for CD and frequently precedes the onset of CD symptoms. Children with an early onset of CD symptoms, with at least one symptom before age 10 years, are at risk for more severe and persistent antisocial behavior continuing into adulthood. Youth with early-onset conduct problems are particularly at risk for ASPD (note that an onset of CD prior to age 15 is part of the diagnostic criteria for ASPD), whereas CD is typically limited to adolescence when youth's CD symptoms begin during adolescence.