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Brief Report
September 16, 2020

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

Author Affiliations
  • 1Evidera Inc, Bethesda, Maryland
  • 2Center for Chronic Pruritus, Department of Dermatology, University Hospital Münster, Münster, Germany
  • 3Menlo Therapeutics Inc, Redwood City, California
JAMA Dermatol. Published online September 16, 2020. doi:10.1001/jamadermatol.2020.3071
Key Points

Question  Is the itch numeric rating scale an appropriate measurement tool for quantifying the extent and intensity of pruritus associated with prurigo nodularis?

Findings  This secondary analysis of a randomized clinical trial of 123 participants found that the Worst Itch Numeric Rating Scale and Average Itch Numeric Rating Scale are acceptable measures of pruritus for patients with prurigo nodularis with good evidence of validity and reliability. This was demonstrated by analyzing test-retest reliability, construct validity, sensitivity to change, and thresholds for meaningful change.

Meaning  The Worst Itch Numeric Rating Scale and Average Itch Numeric Rating Scale should be considered acceptable tools for assessing pruritus in future clinical trials of prurigo nodularis.

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

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