The revised Green et al., Paranoid Thoughts Scale (R-GPTS): psychometric properties, severity ranges, and clinical cut-offs

BACKGROUND: The Green et al., Paranoid Thoughts Scale (GPTS) – comprising two 16-item scales assessing ideas of reference (Part A) and ideas of persecution (Part B) – was developed over a decade ago. Our aim was to conduct the first large-scale psychometric evaluation. METHODS: In total, 10 551 individuals provided GPTS data. Four hundred and twenty-two patients with psychosis and 805 non-clinical individuals completed GPTS Parts A and B. An additional 1743 patients with psychosis and 7581 non-clinical individuals completed GPTS Part B. Factor analysis, item response theory, and receiver operating characteristic analyses were conducted. RESULTS: The original two-factor structure of the GPTS had an inadequate model fit: Part A did not form a unidimensional scale and multiple items were locally dependant. A Revised-GPTS (R-GPTS) was formed, comprising eight-item ideas of reference and 10-item ideas of persecution subscales, which had an excellent model fit. All items in the new Reference (a = 2.09–3.67) and Persecution (a = 2.37–4.38) scales were strongly discriminative of shifts in paranoia and had high reliability across the spectrum of severity (a > 0.90). The R-GPTS score ranges are: average (Reference: 0–9; Persecution: 0–4); elevated (Reference: 10–15; Persecution: 5–10); moderately severe (Reference: 16–20; Persecution:11–17); severe (Reference: 21–24; Persecution: 18–27); and very severe (Reference: 25+; Persecution: 28+). Recommended cut-offs on the persecution scale are 11 to discriminate clinical levels of persecutory ideation and 18 for a likely persecutory delusion. CONCLUSIONS: The psychometric evaluation indicated a need to improve the GPTS. The R-GPTS is a more precise measure, has excellent psychometric properties, and is recommended for future studies of paranoia.