Worst itch numerical rating scale for prurigo nodularis: a psychometric evaluation

Background Study TR03 evaluated the safety and efficacy of nalbuphine ER for prurigo nodularis (PN; NCT02174419). Objective We conducted supplementary analyses to assess the psychometric properties of the Worst Itch Numeric Rating Scale (WI-NRS), the TR03 primary endpoint. Methods Study TR03 was a double-blind, placebo-controlled, phase 2 trial in PN patients with documented scores >= 5 on the WI-NRS (0 [no itch]-10 [worst itch imaginable]) on >= 5 of 7 days before baseline. Using TR03 data, the WI-NRS's psychometric properties, including reliability, validity and ability to detect change, were evaluated. A responder threshold was estimated to facilitate interpretation of WI-NRS score changes. Results Amongst 62 treated patients, improvements in mean [SD] (median) WI-NRS scores were observed between baseline (8.2 [1.21] (8.1)) and week 10 (5.8 [2.43] (6.0)). The WI-NRS had an intraclass correlation coefficient of 0.96 (95% confidence interval, 0.93-0.98) in 42 patients who had stable Itch verbal rating scale (VRS) scores from week 9-10, supporting strong test-retest reliability. Construct validity was supported, with strong correlations at week 10 with Average Itch NRS (r = 0.87) and Itch VRS single-day/weekly mean scores (r = 0.81/0.89) and moderate correlations with ItchyQoL (TM) total/domain scores (r = 0.41-0.43). The WI-NRS discriminated between predefined severity subgroups based on the Itch VRS and detected changes in itching severity (effect-size estimate: -2.05; standardized response mean: -1.21). An anchor-based threshold based on a two-category improvement in the single-day Itch VRS suggests a responder threshold of >= 3.8 points (similar to 40% improvement). Conclusions The WI-NRS demonstrates good measurement properties, supporting its use in evaluating treatment change in PN.

Automated summary

This study evaluated the safety and efficacy of nalbuphine ER for prurigo nodularis and assessed the psychometric properties of the Worst Itch Numeric Rating Scale (WI-NRS). The results showed that WI-NRS has good measurement properties and can be used to evaluate treatment change in PN.