Importance: Prurigo nodularis (PN) is a debilitating skin disease characterized by the hallmark symptom of chronic itch; the intensity of itch in PN was assessed using the Worst Itch Numeric Rating Scale (WI-NRS) to evaluate the primary efficacy end point of 2 recent phase 3 studies of dupilumab treatment for PN.
Objective: To validate the psychometric properties and to determine the clinically meaningful improvement threshold for WI-NRS in patients with moderate to severe PN.
Design, setting, and participants: In this secondary analysis of the PRIME and PRIME2 trials, content validity of WI-NRS was assessed through in-depth patient interviews. Psychometric assessments used pooled data from masked, intention-to-treat (ITT) patients with PN from randomized, double-masked, and placebo-controlled studies. Psychometric assessments included test-retest reliability, construct validity, known-groups validity, and sensitivity to change in adult patients with moderate-to-severe PN. Thresholds for meaningful within-patient improvement in the WI-NRS score were determined using anchor and distribution-based approaches. Data were analyzed after completion of each study, December 2019 to November 2021 for PRIME and January 2020 to August 2021 for PRIME2.
Exposures: Dupilumab (300 mg) or placebo subcutaneously every 2 weeks for 24 weeks.
Main outcomes and measures: WI-NRS score at specified time points up to 24 weeks after randomization.
Results: A total of 20 patients were included across the 2 studies (mean [SD] age, 49.3 [17.2] years; 11 female [55%]); 311 patients were included in the pooled intention-to-treat analysis (mean [SD] age, 49.5 [16.1] years; 203 female [65.3%]). The WI-NRS questions (20 of 20 patients), recall period (19 of 20 patients), and response scale (20 of 20 patients) were easy to understand and relevant for patients with PN. Adequate test-retest reliability was observed between screening and baseline (intraclass correlation coefficient = 0.72, using Patient Global Impression of Severity [PGIS] to define stable patients). Convergent and discriminant validity was supported by moderate to strong correlations (absolute r range = 0.34-0.73) with other conceptually related measures and weaker correlations (absolute r range = 0.06-0.32) with less-related measures, respectively. WI-NRS was sensitive to change, as demonstrated by differences in change from baseline among groups (per PGIS change and PGI of Change [PGIC]). Using anchor-based approach with PGIS and PGIC, the clinically meaningful improvement threshold was 4 points (range, 3.0-4.5), which was also supported by distribution-based methods.
Conclusion and relevance: This study found that WI-NRS may be a fit-for-purpose instrument to support efficacy end points measuring the intensity of itching in adults with PN.
Trial registration: NCT04183335 (PRIME) and NCT04202679 (PRIME2).