There are no blood tests, imaging techniques, or other laboratory procedures that can reliably diagnose a mental illness. Thus, the diagnosis of mental illness is always a judgment or an interpretation by an observer based on the speech, ideas, behaviors, and experiences of the patient. (Diagnosis Symptoms Mental Illness)
For the most part, mental health professionals determine the presence of mental illness in an individual by conducting an interview intended to reveal symptoms of abnormal behavior. That is, the professional asks the patient questions about his or her mental state: “Do you hear voices of people who are not with you?” “Have you felt depressed or lost interest in most activities?” “Have you experienced a marked increase or decrease in your appetite?” “Have you been sleeping less than normal?” “Are you easily distracted?” The answers to these questions will suggest other questions. Eventually, the clinician will feel that he or she has enough information to determine whether the patient is suffering from a mental illness and, if so, to make a diagnosis. (Diagnosis Symptoms Mental Illness)
The process of diagnosis is not as simple as it might seem. Patients often have difficulty remembering symptoms or feel reluctant to talk about their fantasies, sex life, or use of drugs and alcohol. Many patients suffer from more than one disorder at a time—for example, depression and anxiety, or schizophrenia and depression—and determining which symptoms constitute the primary problem is complex. In addition, symptoms may not be specific to mental illnesses. For example, brain tumors, malaria, and infections of the central nervous system can produce symptoms that mimic those of psychotic disorders. (Diagnosis Symptoms Mental Illness)
Another problem in diagnosis is that mental health professionals may interpret symptoms differently based on their personal or cultural biases. One study examined this effect by showing 300 American and British psychiatrists videotaped interviews of eight patients with mental illnesses. Although the psychiatrists’ diagnoses substantially agreed for patients with “textbook” cases of schizophrenia, their diagnoses varied widely for patients who had symptoms of both schizophrenia and other disorders, depending on whether the psychiatrist was American or British. The risk of misdiagnosis is even greater when the mental health professional and the patient come from different cultural groups. (Diagnosis Symptoms Mental Illness)