Worst Itch Numeric Rating Scale proves valid, reliable for prurigo nodularis
Key takeaways:
- Patients reported that the instrument’s questions, recall period and response scale were easy to understand.
- A psychometric analysis showed that the tool also had adequate test-retest reliability.
A new study found that the Worst Itch Numeric Rating Scale is a valid and reliable instrument to capture itch data of patients with prurigo nodularis.
“The Worst Itch Numeric Rating Scale (WI-NRS) is a single-item, patient-reported questionnaire designed to measure an individual’s ‘worst itch’ in the past 24 hours on an 11-point rating scale,” Shawn G. Kwatra, MD, professor and chair of dermatology at University of Maryland School of Medicine, and colleagues wrote. “The psychometric properties of WI-NRS for dermatologic conditions, such as psoriasis and [atopic dermatitis], have been well documented; however, evidence supporting its validation in [prurigo nodularis (PN)] is limited.”
In this study, the authors evaluated whether WI-NRS is an efficient tool to measure itch in PN through qualitative interviews with 20 patients. The researchers also included 311 patients in a pooled intention-to-treat, psychometric analysis of two phase 3 trials, PRIME and PRIME2.
Results from the interviews showed that the WI-NRS questions, recall period and response scale were easy to understand and relevant for patients with PN. According to the psychometric analysis, adequate test-retest reliability was reported in 224 stable patients between screening and baseline (intraclass correlation coefficient [ICC] = 0.72; 95% CI, 0.63-0.79) and weeks 8 to 12 (ICC = 0.88; 95% CI, 0.82-0.92).
Moderate to strong correlations were seen between the WI-NRS scores and similar constructs scores such as the Sleep-NRS (absolute correlation coefficient [r] = 0.41). The analysis also showed that the scale was able to distinguish between patients with different severity levels and quality of life effects.
Additionally, changes in the WI-NRS scores were moderately to strongly correlated to changes in Skin Pain-NRS (r = 0.76) and Sleep-NRS (r = 0.40) from baseline to week 12.
The study found a clinically meaningful improvement threshold of 4 points (range, 3-4.5).
“This study confirms the psychometric and content validity of WI-NRS, thereby establishing it to be a clear, comprehensive and relevant tool to measure itch in PN,” the authors concluded.