Psychometric validation of the Psoriasis Symptom Scale, Functional Assessment of Chronic Illness Therapy-Fatigue and pain-Visual Analogue Scale in patients with generalized pustular psoriasis

Abstract

Background: Generalized pustular psoriasis (GPP) is a rare, chronic, inflammatory skin disease associated with considerable patient burden. The Psoriasis Symptom Scale (PSS), Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue) and pain-Visual Analogue Scale (pain-VAS) are patient-reported outcomes (PROs) that have not yet been validated in patients with GPP.

Objectives: To evaluate the psychometric properties of the PSS, FACIT-Fatigue and pain-VAS using data from Effisayil 1, a randomised trial of spesolimab in patients with moderate-to-severe GPP.

Methods: Inter-item correlations and confirmatory factor analysis (CFA) were performed using Week 1 data. Internal consistency was assessed with Cronbach's α coefficient using baseline and Week 1 data. Test-retest reliability was assessed using intraclass correlation coefficients (ICCs); change data for the GPP Physician Global Assessment total score and pustulation subscore were used to define a stable population. Convergent validity was assessed at baseline and Week 1 using Spearman's rank-order correlations. Known-groups validity was measured by analysis of variance using Week 1 data. Ability to detect change from baseline to Week 1 was evaluated by analysis of covariance.

Results: Inter-item and item-to-total correlations were moderate or strong for most PSS and FACIT-Fatigue items. CFA demonstrated the unidimensionality of the PSS and FACIT-Fatigue, with high factor loadings for most items (PSS range, 0.75-0.94; FACIT-Fatigue range, 0.11-0.93) and acceptable fit statistics. Both scores demonstrated internal consistency (Cronbach's α, 0.71 and 0.95, respectively). The PSS, FACIT-Fatigue and pain-VAS demonstrated test-retest reliability (ICCs ≥0.70) and good evidence of convergent validity. Furthermore, the PROs could differentiate between known groups of varying symptom severity (range, p < 0.0001-0.0225) and detect changes in symptom severity from baseline to Week 1 (range, p < 0.0001-0.0002).

Conclusions: Overall, these results support the reliability, validity and ability to detect change of the PSS, FACIT-Fatigue and pain-VAS as PROs in patients with GPP.