Initial development and pragmatic clinical validation of a static disease severity instrument for pyoderma gangrenosum: Investigator Global Assessment for PG (IGAPg).

Background: Pyoderma gangrenosum (PG) is a rare neutrophilic ulcerative dermatosis with no FDA-approved therapies and limited validated outcome measures. Investigator's Global Assessments (IGAs) are widely used in dermatology but often lack objective criteria, robust validation, and comparability across studies. There remains a critical need for a PG-specific, standardized, and validated severity instrument. Methods: We developed and conducted initial validation of the Investigator Global Assessment for Pyoderma Gangrenosum (IGAPg©), a novel PG-specific IGA. An international multistakeholder panel guided development, with a core team designing the instrument based on prior research identifying key objective clinical features of PG severity. The IGAPg© incorporates ulcer depth, drainage, discoloration, and undermining, with ulcer location and extent used to resolve indeterminate cases. Standardized rater training materials were created. Construct validity was assessed in 36 patients evaluated by an expert PG clinician, with correlations to Patient Global Assessment (PGA), Skindex-Mini, and numeric rating scales (NRS) for 24-hour and 7-day pain. Inter-rater reliability was evaluated in a subset of 26 patients assessed independently by five raters using a two-way random-effects intraclass correlation coefficient (ICC [2,1]) within a linear mixed-effects model. Result: IGAPg© scores demonstrated strong correlation with PGA when assessed by an expert PG dermatologist (Pearson's r = 0.73) and when averaged across all raters (r = 0.69). Moderate correlations were observed with Skindex-Mini (r = 0.49), 24-hour pain NRS (r = 0.48), and 7-day pain NRS (r = 0.52), consistent with differing constructs measured. Inter-rater reliability was high (ICC = 0.76). Raters reported the instrument to be comprehensive, comprehensible, and efficient to administer. Reliability and construct validity metrics are summarized in Table 1. Conclusions: Despite limitations including modest sample size and rater homogeneity, the IGAPg© demonstrated strong construct validity and high inter-rater reliability. Designed for dermatologists and trainees familiar with PG morphology, the IGAPg© represents a promising PG-specific outcome measure for clinical research and therapeutic trials. Future work will focus on refining training materials and expanding validation with structured patient-investigator engagement.