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

Abstract

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.

Conflict of interest statement

Conflict of Interest Disclosures: Dr Kimel received research funding support from Amgen and Menlo Therapeutics. Dr Zeidler has received speaker honoraria/travel fees from Beiersdorf and Dermasence outside the submitted work. Dr Kwon is a former employee of Menlo Therapeutics. Dr Revicki reported grants and personal fees from Takeda during the conduct of the study; grants and personal fees from Amgen and Allergan, grants from Bristol Myers Squibb, and personal fees from Celgene and Alphasigma outside the submitted work; and research funding support from Amgen, Boehringer-Ingelheim, and Menlo Therapeutics. Dr Ständer is an investigator for Dermasence, Galderma, Kiniksa Pharmaceuticals, Menlo Therapeutics, Novartis, Sanofi, Trevi Therapeutics, and Vanda Pharmaceuticals; is a consultant and/or member of the advisory board for Almirall, Bayer, Beiersdorf, BELLUS Health, Bionorica, Cara Therapeutics, Celgene, Clexio Biosciences, DS Biopharma, Galderma, Menlo Therapeutics, Novartis, Perrigo, and Trevi Therapeutics; received other financial support from University Hospital Münster, personal fees from Sonja Ständer, and grants from University Hospital Münster during the conduct of the study; and received personal fees from Cara, Galderma S.A., Kiniksa, Pfizer, Sanofi R&D, and Beiersdorf AG, and grants and personal fees from Trevi outside the submitted work.

Figures

Figure.. Tests of Known Validity for Worst Itch Numeric Rating Scale (WI-NRS) and Average Itch Numeric Rating Scale (AI-NRS) With Measures of Itch, Quality of Life, and Prurigo Nodularis Status

eVRS indicates electronic Verbal Rating Scale; ItchyQoL, the pruritus-specific quality-of-life rating instrument ItchyQoL; DLQI, Dermatology Life Quality Index; vVRS indicates visit Verbal Rating Scale. Weekly mean itch numeric rating scale (NRS) and electronic verbal rating scale for itch severity scores from the previous week were used to determine WI-NRS and AI-NRS. Both WI-NRS and AI-NRS were assessed using analysis of variance models. A, Mean NRS with visit verbal rating scale (itching) from the visit Patient Global Assessment. The none and mild categories were combined due to small sample size for the none category (n = 1 or 2). B, Mean NRS with electronic verbal rating scale for itch severity. C, Mean NRS with ItchyQoL. D, Mean NRS with Dermatology Life Quality Index (DLQI). E, Mean NRS with percentage of prurigo lesions with excoriations.