Psychiatrists seek to provide a medical diagnosis of individuals by an assessment of symptoms, signs and impairment associated with particular types of mental disorder. Other mental health professionals, such as clinical psychologists, may or may not apply the same diagnostic categories to their clinical formulation of a client's difficulties and circumstances. The majority of mental health problems are, at least initially, assessed and treated by family physicians (in the UK general practitioners) during consultations, who may refer a patient on for more specialist diagnosis in acute or chronic cases.

Routine diagnostic practice in mental health services typically involves an interview known as a mental status examination, where evaluations are made of appearance and behavior, self-reported symptoms, mental health history, and current life circumstances. The views of other professionals, relatives or other third parties may be taken into account. A physical examination to check for ill health or the effects of medications or other drugs may be conducted. Psychological testing is sometimes used via paper-and-pen or computerized questionnaires, which may include algorithms based on ticking off standardized diagnostic criteria, and in rare specialist cases neuroimaging tests may be requested, but such methods are more commonly found in research studies than routine clinical practice.

Time and budgetary constraints often limit practicing psychiatrists from conducting more thorough diagnostic evaluations. It has been found that most clinicians evaluate patients using an unstructured, open-ended approach, with limited training in evidence-based assessment methods, and that inaccurate diagnosis may be common in routine practice. In addition, comorbidity is very common in psychiatric diagnosis, where the same person meets the criteria for more than one disorder. On the other hand, a person may have several different difficulties only some of which meet the criteria for being diagnosed. There may be specific problems with accurate diagnosis in developing countries.

More structured approaches are being increasingly used to measure levels of mental illness.

- HoNOS is the most widely used measure in English mental health services, being used by at least 61 trusts. In HoNOS a score of 0–4 is given for each of 12 factors, based on functional living capacity. Research has been supportive of HoNOS, although some questions have been asked about whether it provides adequate coverage of the range and complexity of mental illness problems, and whether the fact that often only 3 of the 12 scales vary over time gives enough subtlety to accurately measure outcomes of treatment.

The word '' means 'substitute'. It is coded in the DSM-IV under Factitious Disorder NOS (not otherwise specified). Münchausen by proxy is the involuntary use of another individual to play the patient role. For example, false symptoms are produced in children by the caregivers or parents, to produce the appearance of illness, or they may give misleading medical histories about their children. The parent may falsify the child's medical history or tamper with laboratory tests to make the child appear sick. Occasionally, in Münchausen by proxy, the caregiver actually injures the child or makes it sick to ensure that the child is treated. For instance, a father whose son is coeliac might knowingly introduce gluten into the diet. Such parents may be validated by the attention that they receive from having a sick child.

Since the 1980s, Paula Caplan has been concerned about the subjectivity of psychiatric diagnosis, and people being arbitrarily “slapped with a psychiatric label.” Caplan says because psychiatric diagnosis is unregulated, doctors are not required to spend much time interviewing patients or to seek a second opinion. The Diagnostic and Statistical Manual of Mental Disorders can lead a psychiatrist to focus on narrow checklists of symptoms, with little consideration of what is actually causing the patient’s problems. So, according to Caplan, getting a psychiatric diagnosis and label often stands in the way of recovery.

In 2013, psychiatrist Allen Frances wrote a paper entitled "The New Crisis of Confidence in Psychiatric Diagnosis", which said that "psychiatric diagnosis… still relies exclusively on fallible subjective judgments rather than objective biological tests." Frances was also concerned about "unpredictable overdiagnosis." For many years, marginalized psychiatrists (such as Peter Breggin, Thomas Szasz) and outside critics (such as Stuart A. Kirk) have "been accusing psychiatry of engaging in the systematic medicalization of normality." More recently these concerns have come from insiders who have worked for and promoted the American Psychiatric Association (e.g., Robert Spitzer, Allen Frances). A 2002 editorial in the "British Medical Journal" warned of inappropriate medicalization leading to disease mongering, where the boundaries of the definition of illnesses are expanded to include personal problems as medical problems or risks of diseases are emphasized to broaden the market for medications.

Perhaps due to their perceived rarity, the dissociative disorders (including DID) were not initially included in the Structured Clinical Interview for DSM-IV (SCID), which is designed to make psychiatric diagnoses more rigorous and reliable. Instead, shortly after the publication of the initial SCID a freestanding protocol for dissociative disorders (SCID-D) was published. This interview takes about 30 to 90 minutes depending on the subject's experiences. An alternative diagnostic instrument, the Dissociative Disorders Interview Schedule, also exists but the SCID-D is generally considered superior. The Dissociative Disorders Interview Schedule (DDIS) is a highly structured interview that discriminates among various DSM-IV diagnoses. The DDIS can usually be administered in 30–45 minutes.

Other questionnaires include the Dissociative Experiences Scale (DES), Perceptual Alterations Scale, Questionnaire on Experiences of Dissociation, Dissociation Questionnaire, and the Mini-SCIDD. All are strongly intercorrelated and except the Mini-SCIDD, all incorporate absorption, a normal part of personality involving narrowing or broadening of attention. The DES is a simple, quick, and validated questionnaire that has been widely used to screen for dissociative symptoms, with variations for children and adolescents. Tests such as the DES provide a quick method of screening subjects so that the more time-consuming structured clinical interview can be used in the group with high DES scores. Depending on where the cutoff is set, people who would subsequently be diagnosed can be missed. An early recommended cutoff was 15-20. The reliability of the DES in non-clinical samples has been questioned.

No true psychiatric medications are prescribed for factitious disorder. However, selective serotonin reuptake inhibitors (SSRIs) can help manage underlying problems. Medicines such as SSRIs that are used to treat mood disorders can be used to treat FD, as a mood disorder may be the underlying cause of FD. Some authors (such as Prior and Gordon 1997) also report good responses to antipsychotic drugs such as Pimozide. Family therapy can also help. In such therapy, families are helped to better understand patients (the individual in the family with FD) and that person's need for attention.

In this therapeutic setting, the family is urged not to condone or reward the FD individual's behavior. This form of treatment can be unsuccessful if the family is uncooperative or displays signs of denial and/or antisocial disorder. Psychotherapy is another method used to treat the disorder. These sessions should focus on the psychiatrist's establishing and maintaining a relationship with the patient. Such a relationship may help to contain symptoms of FD. Monitoring is also a form that may be indicated for the FD patient's own good; FD (especially proxy) can be detrimental to an individual's health—if they are, in fact, causing true physiological illnesses. Even faked illnesses/injuries can be dangerous, and might be monitored for fear that unnecessary surgery may subsequently be performed.

The fourth, revised edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) diagnoses DID according to the diagnostic criteria found in section 300.14 (dissociative disorders). It has also been found difficult to diagnose the disorder in the first place, due to there not being a universal agreement of the definition of dissociation. The criteria require that an adult be recurrently controlled by two or more discrete identities or personality states, accompanied by memory lapses for important information that is not caused by alcohol, drugs or medications and other medical conditions such as complex partial seizures. While otherwise similar, the diagnostic criteria for children also specifies symptoms must not be confused with imaginative play. Diagnosis is normally performed by a clinically trained mental health professional such as a psychiatrist or psychologist through clinical evaluation, interviews with family and friends, and consideration of other ancillary material. Specially designed interviews (such as the SCID-D) and personality assessment tools may be used in the evaluation as well. Since most of the symptoms depend on self-report and are not concrete and observable, there is a degree of subjectivity in making the diagnosis. People are often disinclined to seek treatment, especially since their symptoms may not be taken seriously; thus dissociative disorders have been referred to as "diseases of hiddenness".

The diagnosis has been criticized by supporters of therapy as a cause or the sociocognitive hypothesis as they believe it is a culture-bound and often health care induced condition. The social cues involved in diagnosis may be instrumental in shaping patient behavior or attribution, such that symptoms within one context may be linked to DID, while in another time or place the diagnosis could have been something other than DID. Other researchers disagree and argue that the existence of the condition and its inclusion in the DSM is supported by multiple lines of reliable evidence, with diagnostic criteria allowing it to be clearly discriminated from conditions it is often mistaken for (schizophrenia, borderline personality disorder, and seizure disorder). That a large proportion of cases are diagnosed by specific health care providers, and that symptoms have been created in nonclinical research subjects given appropriate cueing has been suggested as evidence that a small number of clinicians who specialize in DID are responsible for the creation of alters through therapy.

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.

Because there is uncertainty in treating suspected factitious disorder imposed on self, some advocate that health care providers first explicitly rule out the possibility that the person has another early-stage disease. Then they may take a careful history and seek medical records to look for early deprivation, childhood abuse, or mental illness. If a person is at risk to themself, psychiatric hospitalization may be initiated.

Healthcare providers may consider working with mental health specialists to help treat the underlying mood or disorder as well as to avoid countertransference. Therapeutic and medical treatment may center on the underlying psychiatric disorder: a mood disorder, an anxiety disorder, or borderline personality disorder. The patient's prognosis depends upon the category under which the underlying disorder falls; depression and anxiety, for example, generally respond well to medication and/or cognitive behavioral therapy, whereas borderline personality disorder, like all personality disorders, is presumed to be pervasive and more stable over time, and thus offers a worse prognosis.

People affected may have multiple scars on their abdomen due to repeated "emergency" operations.

Diagnosing factitious disorder imposed on self requires a clinical assessment. Clinicians should be aware that those presenting with symptoms (or persons reporting for that person) may malinger, and caution should be taken to ensure there is evidence for a diagnosis. Lab tests may be required, including complete blood count (CBC), urine toxicology, drug levels from blood, cultures, coagulation tests, assays for thyroid function, or DNA typing. In some cases CT scan, magnetic resonance imaging, psychological testing, electroencephalography, or electrocardiography may also be employed. A summary of more common and reported cases of factitious disorder (Munchausen syndrome), and the laboratory tests used to differentiate these from authentic disease is provided below:

There are several symptoms that together point to factitious disorder, including frequent hospitalizations, knowledge of several illnesses, frequently requesting medication such as pain killers, openness to extensive surgery, few or no visitors during hospitalizations, and exaggerated or fabricated stories about several medical problems. Factitious disorder should not be confused with hypochondria, as people with factitious disorder syndrome do not really believe they are sick; they only want to be sick, and thus fabricate the symptoms of an illness. It is also not the same as pretending to be sick for personal benefit such as being excused from work or school.

People may fake their symptoms in multiple ways. Other than making up past medical histories and faking illnesses, people might inflict harm on themselves by consuming laxatives or other substances, self-inflicting injury to induce bleeding, and altering laboratory samples.” Many of these conditions do not have clearly observable or diagnostic symptoms and sometimes the syndrome will go undetected because patients will fabricate identities when visiting the hospital several times. Factitious disorder has several complications, as these people will go to great lengths to fake their illness. Severe health problems, serious injuries, loss of limbs or organs, and even death are possible complications.

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.

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.

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."

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.

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.

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.

The rate in school age children is thought to be about 1.5%, compared with an estimated 5.3% for ADHD.

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.

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.

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.

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.

The psychological mechanism of conversion can be the most difficult aspect of a conversion diagnosis. Even if there is a clear antecedent trauma or other possible psychological trigger, it is still not clear exactly how this gives rise to the symptoms observed. Patients with medically unexplained neurological symptoms may not have any psychological stressor, hence the use of the term "functional neurological symptom disorder" in DSM-V as opposed to "conversion disorder", and DSM-V's removal of the need for a psychological trigger.

As it has already been mentioned, patients with organic personality disorder show a wide variety of sudden behavioural changes and dysfunctions. There are not a lot of information about the treatment of this mental health disorder. The pharmacological approach is the most common therapy among patients with organic personality disorder. However, the choice of drug therapy relies on the seriousness of patient's situation and what symptoms are shown. The choice and administration of specific drugs contribute to the reduction of symptoms of organic personality disorder. For this reason, it is crucial for patients' treatment to be assessed by clinical psychologists and psychiatrists before the administration of drug.

Additionally, the dysfunctions in expression of behaviour of patients with organic personality disorder and the development of symptom of irritability, which are caused by aggressive and self-injurious behaviours, can be dealt with the administration of carbamazepine. Moreover, the symptoms of this disorder can be decreased by the administration of valproic acid. Also, emotional irritability and signs of depression can be dealt with the use of nortriptyline and low-dose thioridazine. Except from the symptom of irritability, patients express aggressive behaviours. At the onset of drug therapy for effective treatment of anger and aggression, the drug of carbamazepine, phenobarbital, benztropine and haloperidol can be administrated in order to reduce the symptoms of patients with organic personality disorder. In addition, the use of propranolol may decrease the frequent behaviours of rage attacks.

Finally, it is important for patients to take part in psychotherapy sessions during the period of drug therapy. In this way, there is prevention and patients can be protected by negative effects of drugs on their organism and their behaviour. Furthermore, the clinicians can provide useful and helpful support to patients during these psychotherapy sessions. Thus, the combination of drug therapy with psychotherapy can lead to the reduction of symptoms of this disorder and the improvement of patients' situation.

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.

The ICD-10 includes a diagnostic guideline for the wide group of personality and behavioural disorders. However, every disorder has its own diagnostic criteria. In case of the organic personality disorder, patient has to show at least three of the following diagnostic criteria over a six or more months period. organic personality disorder is associated with a large variety of symptoms, such as deficits in cognitive function, dysfunctional behaviours, psychosis, neurosis, emotional changes, alterations in expression function and irritability. Patients with organic personality disorder can present emotional lability that means their emotional expressions are unstable and fluctuating. In addition, patients show reduction in ability of perseverance with their goals and they express disinhibited behaviours, which are characterised by inappropriate sexual and antisocial actions. For instance, patients can show dissocial behaviours, like stealing. Moreover, according to diagnostic guideline of ICD-10, patients can suffer from cognitive disturbances and they present signs of suspiciousness and paranoid ideas. Additionally, patients may present alteration in process of language production that means there are changes in language rate and flow. Furthermore, patients may show changes in their sexual preference and hyposexuality symptoms.

Another common feature of personality of patients with organic personality disorder is their dysfunctional and maladaptive behaviour that causes serious problems in these patients, because they face problems with pursuit and achievement of their goals. It is worth to be mentioned that patients with organic personality disorder express a feeling of unreasonable satisfaction and euphoria. Also, the patients show aggressive behaviours sometimes and these serious dysfunctions in their behaviour can have effects on their life and their relationships with other people. Specifically patients show intense signs of anger and aggression because of their inability to handle their impulses. The type of this aggression is called "impulsive aggression". Furthermore, it is worth to be mentioned that the pattern of organic personality disorder presents some similarities with pattern of temporal lobe epilepsy (TLE). Specifically patients who suffer from this chronic disorder type of epilepsy, express aggressive behaviours, likewise it happens to patients with organic personality disorder. Another similar symptom between Temporal lobe epilepsy and organic personality disorder is the epileptic seizure. The symptom of epileptic seizure has influence on patients' personality that means it causes behavioural alterations". The Temporal lobe epilepsy (TLE) is associated with the hyperexcitability of the medial temporal lobe (MTL) of patients. Finally, patients with organic personality disorder may present similar symptoms with patients, who suffer from the Huntington's disease as well. The symptoms of apathy and irritability are common between these two groups of patients.

Hospitalization may be necessary during the acute phase of symptoms, and psychiatric care if the patient is a danger to self or others. A neurological consult is advised to rule out any organic cause.