If the doctor finds no physical reason for the symptoms, he or she might refer the person to a psychiatrist or psychologist. They use specially designed interview and assessment tools to evaluate a person for a psychotic disorder. The doctor or therapist bases his or her diagnosis on the person's report of symptoms, and his or her observation of the person's attitude and behavior. The doctor or therapist then determines if the person's symptoms point to a specific disorder. The doctor will make an official diagnosis of delusional disorder if a person has non-bizarre delusions for at least one month and does not have symptoms of other psychotic disorders, such as schizophrenia.