The predictive validity of a diagnosis of schizophrenia. A report from the International Study of Schizophrenia (ISoS) coordinated by the World Health Organization and the Department of Psychiatry, University of Nottingham

Background Outcome is important in the validation of psychiatric diagnosis, as most disorders lack clinicopathological correlates. We describe the predictive validity of four definitions of schizophrenia (DSM-III-R, ICD-10, ICD-9 and CATEGO S+), in a representative cohort of patients selected during their first episode of psychosis.
Method Each definition of schizophrenia was applied to 99 patients. Their respective ability to predict 13-year outcome (Global Assessment of Functioning scales) was assessed.
Results DSM-III-R and ICD-10 diagnoses of schizophrenia have high predictive validity for long-term outcome, and both provide relatively stable diagnoses. ICD-9 is reasonably good at predicting disability, but not symptoms, and CATEGO S+ showed no predictive validity. Adding six-month duration criteria to ICD-10, ICD-9 and CATEGO S+ improved their predictive validity, and removing the six-month duration criterion from DSM-III-R commensurately reduced predictive validity.
Conclusions Modern diagnostic systems (DSM-III-R and ICD-10) have high predictive validity, and are superior to ICD-9. The six-month duration criterion of DSM-III-R schizophrenia accounts for its predictive validity and stability over 13 years, but restricts its use in first-episode studies. The one-month duration criterion of ICD-10 is less restrictive, without major compromises in predictive validity or stability.