Summary
Background
Itching is a profoundly distressing symptom for many patients with psoriasis but 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 worst itching severity due to psoriasis in the past 24 hours.
Methods
Using disease-specific clinician-rated and PRO data from one phase 2 and three phase 3 randomised 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 construct validity of the Itch NRS, large cross-sectional correlations with the Dermatology Quality of Life (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 that 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.
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