Psychometric validation and meaningful change thresholds of the Worst Itching Intensity Numerical Rating Scale for assessing itch in patients with chronic kidney disease-associated pruritus

Abstract

Abstract Background Chronic kidney disease-associated pruritus (CKD-aP) is characterized by persistent itch that often leads to substantially impaired quality of life. The Worst Itching Intensity Numerical Rating Scale (WI-NRS) is a single-item patient-reported outcome measure in which patients indicate the intensity of the worst itching they experienced over the past 24 h. Here, we evaluated the content validity and psychometric properties of the WI-NRS and confirmed the threshold of meaningful change in hemodialysis patients with moderate-to-severe CKD-aP. Methods Content validity interviews were conducted in 23 patients. Psychometric properties of the WI-NRS were assessed using data from one phase 2 (N = 174) and two phase 3 (N = 848) clinical trials investigating an anti-pruritic treatment. Anchor-based methods were used to confirm meaningful within-patient change score thresholds in the phase 3 trial patients and mixed-method exit interviews (N = 70) contributed further insight. Results Content validity interviews indicated patients considered the WI-NRS to be straightforward, comprehensive, and relevant. Test–retest reliability was strong in both trial cohorts (intraclass correlation coefficients > 0.75). Construct validity analyses indicated high correlation between the WI-NRS and other measures of itch. Anchor-based analyses showed a reduction of ≥ 3 points from baseline score represented an appropriate clinically meaningful within-patient change on the WI-NRS. In the exit interviews, all patients with a reduction ≥ 3 points considered the change meaningful. Conclusions The WI-NRS is a reliable, valid, and responsive measure of itch intensity for patients with moderate-to-severe CKD-aP. These results support its use to assess treatment efficacy and in clinical evaluation and management of pruritus in hemodialysis patients.