Glasgow Psychosis Screening tool for use in adults with Intellectual Disabilities (GPS-ID): development and psychometric properties

Background: Prevalence of psychosis is known to be higher in adults with intellectual disabilities (ID) than in the general adult population. However, there have been no attempts to develop a psychosis screening tool specifically for the adult ID population. The present study describes the development and preliminary evaluation of a new measure, the Glasgow Psychosis Screening tool for use in Adults with Intellectual Disabilities (GPS-ID). Method: An item pool was generated following: 1) focus groups with adults with ID and psychosis, and their carers and/or workers; 2) expert input from clinicians. A draft scale was compiled and refined following expert feedback. The new scale, along with the Psychotic Symptom Rating Scales was administered to 20 adults with ID (10 with and 10 without psychosis) and their relative or carers. Results: The GPS-ID total score, self-report subscale and informant rating-subscale differentiated psychosis and non-psychosis groups. The tool had good internal consistency (Cronbach’s α=0.91), and a cut-off score ≥4 yielded high sensitivity (90%) and specificity (100%). The method of tool development supports face and content validity. Criterion validity was not supported. Conclusions: Preliminary investigation of the tool’s psychometric properties is positive, although further investigation is required. The tool is accessible to adults with mild to moderate ID and can be completed in 15-30 minutes. The GPS-ID is not a diagnostic tool, therefore any adult exceeding the cut-off score of ≥4 should receive further assessment.