Before someone can be diagnosed with residual schizophrenia, a doctor must make sure that the person's symptoms aren't being caused by something else, such as depression, illegal drug use, or another physical problem. The DSM (Diagnostic and Statistical Manual of Mental Disorders) has criteria for diagnosing residual schizophrenia, including:
- A lack of positive symptoms such as delusions, hallucinations, jumbled speech, and catatonia (remaining in one place in a stiff position for a long time);
- Continuing negative symptoms such as lack of interest in socializing, personal hygiene, and daily life activities;
- Milder versions of certain schizophrenic symptoms such as odd beliefs and perceptions.
Symptoms need to be experienced for at least six months and create significant problems in the person's life. Doctors will likely also take a family and medical history, conduct blood tests to rule out other disorders, and may have an MRI or CT done. The MRI or CT can't confirm the diagnosis, however, and only provide additional information for medical staff.