Psychometric validation of the generalized pustular psoriasis physician global assessment (GPPGA) and generalized pustular psoriasis area and severity index (GPPASI)

Background: Generalized pustular psoriasis (GPP) is a rare and life-threatening skin disease often accompanied by systemic inflammation. There are currently no standardized or validated GPP-specific measures for assessing severity. Objective: To evaluate the reliability, validity and responder definitions of the Generalized Pustular Psoriasis Physician Global Assessment (GPPGA) and Generalized Pustular Psoriasis Area and Severity Index (GPPASI). Methods: The GPPGA and GPPASI were validated using outcome data from Week 1 of the Effisayil™ 1 study. The psychometric analyses performed included confirmatory factor analysis, item-to-item/item-to-total correlations, internal consistency reliability, test–retest reliability, convergent validity, known-groups validity, responsiveness analysis and responder definition analysis. Results: Using data from this patient cohort (N = 53), confirmatory factor analysis demonstrated unidimensionality of the GPPGA total score (root mean square error of approximation <0.08), and GPPGA item-to-item and item-to-total correlations ranged from 0.58 to 0.90. The GPPGA total score, pustulation subscore and GPPASI total score all demonstrated good test–retest reliability (intraclass correlation coefficient: 0.70, 0.91 and 0.95 respectively), and good evidence of convergent validity. In anchor-based analyses, all three scores were able to detect changes in symptom and disease severity over time; reductions of −1.4, −2.2 and − 12.0 were suggested as clinically meaningful improvement thresholds for the GPPGA total score, GPPGA pustulation subscore and GPPASI total score respectively. Anchor-based analyses also supported the GPPASI 50 as a clinically meaningful threshold for improvement. Conclusions: Overall, our findings indicate that the GPPGA and GPPASI are valid, reliable and responsive measures for the assessment of GPP disease severity, and support their use in informing clinical endpoints in trials in GPP.