The diagnosis of Dissociative identity disorder is defined by criteria in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM). The DSM-II used the term multiple personality disorder, the DSM-III grouped the diagnosis with the other four major dissociative disorders, and the DSM-IV-TR categorizes it as dissociative identity disorder. The ICD-10 continues to list the condition as multiple personality disorder.
The diagnostic criteria in section 300.14 (dissociative disorders) of the DSM-IV require:

• The presence of two or more distinct identity or personality states, each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self.
• At least two of these identities or personality states recurrently take control of the person's behavior.
• Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
• The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during alcohol intoxication) or a general medical condition (e.g., complex partial seizures). In children, the symptoms are not attributable to imaginary playmates or other fantasy play. A patient history, x-rays, blood tests, and other procedures can be used to eliminate the possibility that symptoms are due to traumatic brain injury, medication, sleep deprivation, or intoxicants, all of which can mimic symptoms of DID.
Diagnosis should be performed by a psychiatrist or psychologist who may use specially designed interviews (such as the SCID-D) and personality assessment tools to evaluate a person for a dissociative disorder.
The psychiatric history of individuals diagnosed with DID frequently contain multiple previous diagnoses of various mental disorders and treatment failures.


Screening
The SCID-D may be used to make a diagnosis. This interview takes about 30 to 90 minutes depending on the subject's experiences.
The Dissociative Disorders Interview Schedule (DDIS) is a highly structured interview which discriminates between various DSM-IV diagnoses. The DDIS can usually be administered in 30–45 minutes.
The Dissociative Experiences Scale (DES) is a simple, quick, and validated questionnaire that has been widely used to screen for dissociative symptoms. 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 and in one study a DES with a cutoff of 30 missed 46 percent of the positive SCID-D diagnoses and a cutoff of 20 missed 25%. The reliability of the DES in non-clinical samples has been questioned. There is also a DES scale for children and DES scale for adolescents. One study argued that old and new trauma may interact, causing higher DID item test scores.


Differential diagnoses
Conditions which may present with similar symptoms include borderline personality disorder, and the dissociative conditions of dissociative amnesia and dissociative fugue. The clearest distinction is the lack of discrete formed personalities in these conditions. Malingering may also be considered, and schizophrenia, although those with this last condition will have some form of delusions, hallucinations or thought disorder.

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