Validation of Psychometric Properties of the Itch Numeric Rating Scale for Pruritus Associated With Prurigo Nodularis: A Secondary Analysis of a Randomized Clinical Trial.

Importance: There is an unmet need for psychometrically sound instruments to measure pruritus associated with prurigo nodularis (PN).

Objective: To evaluate the psychometric properties of the itch numeric rating scale (itch NRS), both the Worst Itch Numeric Rating Scale (WI-NRS) and the Average Itch Numeric Rating Scale (AI-NRS).

Design, Setting, And Participants: This secondary analysis is based on a secondary end point of a phase 2 randomized clinical trial of serlopitant for treatment of pruritus associated with PN. This randomized, double-blind, placebo-controlled study was conducted at 15 sites in Germany. Eligible patients were aged 18 to 80 years and had generalized PN for more than 6 weeks that was refractory to previous antipruritic therapies. Patients were required to have a visual analog scale itch score of 7 or higher at screening. Data were collected from July 2014 to June 2016 and analyzed from June 2016 to January 2017.

Main Outcomes And Measures: The itch NRS (AI-NRS and WI-NRS) was correlated together with the following measures: the electronic verbal rating scale (eVRS) for itch self-categorization, average itch visual analog scale (AI-VAS), worst itch visual analog scale (WI-VAS), the pruritus-specific quality-of-life rating instrument ItchyQoL, Dermatology Life Quality Index (DLQI), and Prurigo Activity and Severity Score (items 7b and 7a: percentage healed prurigo lesions and percentage of prurigo lesions with excoriations).

Results: There were 123 participants in this study; the mean (SD) age of participants was 57.3 (11.58) years, and 58 (47.2%) were male. Strong associations (r ≥ 0.5) were observed between itch NRS items (WI-NRS and AI-NRS) and AI-VAS (24 hours) at weeks 2, 4, and 8 (r = 0.72-0.90; P < .001). Similar strong associations were also observed between itch NRS items and WI-VAS (24 hours) and eVRS for itch severity across weeks 2, 4, and 8 (r = 0.65-0.92; all P < .001). Strong correlations were seen between change scores for WI-NRS and WI-VAS and AI-VAS (r = 0.76 and 0.70, respectively; both P < .001). Similar findings were seen for AI-NRS, where correlations between change scores for WI-VAS and AI-VAS were 0.71 and 0.72, respectively (both P < .001). Analyses for the itch NRS items also showed that test-retest reliability was acceptable and provided evidence of acceptable convergent validity based on the eVRS and visit verbal rating score for itch self-categorization, ItchyQoL, and DLQI.

Conclusions And Relevance: Results from this secondary analysis show that the itch NRS items WI-NRS and AI-NRS have good psychometric properties for pruritus associated with PN and should be considered acceptable tools for assessing pruritus in future clinical trials of PN.

Trial Registration: ClinicalTrials.gov Identifier: NCT02196324.