According to the most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), delusion is defined as: “A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes the inconvertible and obvious proof or evidence to the contrary.”
Delusion is diagnosed by a psychiatrist who carries out a thorough examination. For an individual to be diagnosed as delusional, the belief cannot be occurring as a result of using drugs or a general medication and the person must be free of any history of schizophrenia.
In order to diagnose delusion disorder, tests are carried out to detect and rule out symptoms that are consistent with other psychiatric disorders such as schizophrenia, schizoaffective disorder, mania or depression.
In paranoid schizophrenia, the delusions tend to be absurd or bizarre and are accompanied by other schizophrenia symptoms such as disordered thought and auditory hallucination, for example.
Sometimes, delusions of persecution (fear that someone is out to harm their person) are seen in patients with depression and may be associated with a sense of guilt where the person feels they have done something very wrong for which they are being persecuted. Delusions of persecution are also seen in people with mania. Similarly, individuals with paranoid personality disorder inherently distrust and suspect people, which can lead to delusions of persecution.
Several medical conditions such as thyroid disorder, systemic lupus erythematosus and dementia also cause symptoms similar to those seen in individuals with delusion and these conditions also need to be ruled out when diagnosing delusion.
According to the DSM-IV, the symptoms of delusion disorder should be present for at least a month before confirming a diagnosis and according to the ICD-10, the symptoms should be present for at least 3 months for a diagnosis to be confirmed.