Reliability and Validity of the Psoriasis Symptom Inventory in Patients with Psoriatic Arthritis

Objective To evaluate the measurement properties of the Psoriasis Symptom Inventory (PSI) in psoriatic arthritis (PsA). Methods The PSI is an 8-item, patient-reported outcome measure of the severity of psoriasis signs and symptoms. This was a secondary analysis of pooled data from a phase II study evaluating the efficacy of brodalumab in patients with PsA. Unidimensionality and item evaluation were assessed using factor and Rasch analyses. Reliability was assessed using Cronbach's alpha (internal consistency) and intraclass correlation coefficients (ICCs) for PSI scores in patients with stable disease (test-retest). Construct validity was evaluated by correlations between PSI scores and body surface area (BSA) affected by psoriasis and selected Short Form 36 (SF-36) health survey domains. Known-groups validity was evaluated based on BSA severity categories, and the ability to detect change was evaluated based on improvement in the subject's global assessment (SGA). Results The analysis sample (n = 154) was 93.5% white and 63.0% female. The mean ± SD baseline affected BSA was 10.4% ± 15.6%, and age was 52.2 ± 11.5 years. The PSI demonstrated unidimensionality, with good item fit and correctly ordered categories, excellent internal consistency (α = 0.95), good test-retest reliability (total score ICC 0.70; item ICCs range 0.67-0.81), convergent validity based on moderate correlations with BSA (r = 0.50), discriminant validity based on small baseline correlations (r <-0.3) with the SF-36 domains (role-physical, role-emotional, vitality), known groups validity based on significant differences between BSA groups, and responsiveness based on SGA improvements (P < 0.05). Conclusion The PSI demonstrated excellent test-retest and internal consistency reliability and good construct validity in measuring psoriasis signs and symptoms severity in PsA.