Psychometric properties of the Itch Numeric Rating Scale in patients with moderate-to-severe plaque psoriasis

Background: Itching is a profoundly distressing symptom for many patients with psoriasis, but it has not been rigorously studied using validated tools for this condition.

Objectives: This study investigated the psychometric properties of the Itch Numeric Rating Scale (Itch NRS), a single-item patient-reported outcome (PRO) measuring the worst itching severity due to psoriasis in the past 24 h.

Methods: Using disease-specific clinician-rated and PRO data from one phase II and three phase III randomized clinical studies of subjects with moderate-to-severe plaque psoriasis, the Itch NRS was evaluated for test-retest reliability, construct validity and responsiveness. A responder definition was explored using anchor- and distribution-based methods.

Results: Test-retest reliability analyses supported the reproducibility of the measure (intraclass correlation coefficient range 0·71-0·74). To support the construct validity of the Itch NRS, large cross-sectional correlations with the Dermatology Life Quality Index (DLQI) Symptoms and Feelings domain (r ≥ 0·60 at baseline and r ≥ 0·80 at week 12) supported a priori hypotheses, while large correlations (r ≥ 0·71) between changes in Itch NRS scores and changes in DLQI Symptoms and Feelings domain scores from baseline to week 12 established responsiveness. A 4-point change was optimal for demonstrating a level of clinically meaningful improvement in itch severity after 12 weeks of treatment, which corresponds with marked clinical improvements in plaque psoriasis.

Conclusions: The Itch NRS demonstrated sufficient reliability, validity and responsiveness, and appropriate interpretation standards for evaluating change over time in itch severity among patients with moderate-to-severe plaque psoriasis when validated using clinical trial data for this condition.