Since delusional disorder is rare, a doctor should evaluate the possibility that another major illness, such as schizophrenia, a mood disorder or a medical problem, is causing the symptoms. Medical causes should be considered, especially later in life. People who develop dementia (for example, Alzheimer's disease) can become delusional.
Making a diagnosis is more difficult when the affected person conceals his or her thoughts. Because the person is convinced of the reality of his or her ideas, he or she may not want treatment. If the person allows it, conversations with supportive family or friends can help. A general medical evaluation is useful. In a few cases, when a medical or neurological problem is suspected, diagnostic tests such as an electroencephalogram (EEG), magnetic resonance imaging (MRI) or computed tomography (CT) scans may be suggested.