Definition and prevalence of severe and persistent mental illness

Background: There is little consistency in how severe mental illness (SMI) is defined in practice, and no operational definitions. Aims: To test two operationalised definitions, based on the National Institute of Mental Health (1987) definition: the first uses three criteria (diagnosis of psychosis; duration of service contact ≥ 2 years; GAF score ≤ 50), the second only the last two. Method: Annual prevalence rates of SMI in two European catchment areas for each criterion and the criteria combined were calculated. Results: The first definition produced rates of 2,55 and 1.34/1000 in London and Verona, respectively; the second permitted an additional 0.98/1000 non-psychotic disorders to be included in Verona. Conclusions: The three-dimensional definition selects a small group of patients with SMI who have psychotic disorders. The two-dimensional approach allows estimates of SMI prevalence rates which include all forms of mental disorder. Declaration of interest. Funding provided by the University of Verona and the Bethlem and Maudsley NHS Trust.