Reliability, Validity, and the Ability to Detect Change of the Psoriasis Symptom Scale (PSS) in Patients with Plaque Psoriasis.

Introduction: The primary objective of the study was to evaluate the measurement properties of the patient-reported four-item Psoriasis Symptom Scale (PSS). Methods: Analysis of phase-III data on the efficacy of risankizumab to assess psychometric characteristics of the PSS in patients with moderate-to-severe psoriasis. Results: PSS items had a good range of symptom severity coverage. The PSS had good test-retest reliability (ICCs >0.90). Convergent and discriminant validity was indicated by moderate-to-strong correlations between the PSS and Dermatology Life Quality Index (DLQI), PSS pain item and EQ-5D pain/discomfort item at week 12 (0.63), and moderate negative correlation with EQ-Visual Analog Scale score at week 12 (-0.37). Known groups validity demonstrated as mean PSS total scores varied by Psoriasis Area and Severity Index (PASI) and Static Physician's Global Assessment (sPGA) defined groups (p < .0001). PSS total scores were responsive to changes in PASI score (p < .0001) and sPGA (p < .0001). PSS minimal, clinical, and meaningful change is estimated to be 1 to 2 points; a preliminary responder definition is a total change score of 3 to 4 points. Conclusions: The PSS is a short, valid unidimensional measure of psoriasis symptom severity, well suited for use in clinical trials.