Psychometric validation and meaningful change thresholds of the Skindex-10 questionnaire and 5-D Itch scale for assessing itch in patients with chronic kidney disease-associated pruritus

Abstract Background The Skindex-10 questionnaire and 5-D Itch scale are patient-reported outcome measures used to evaluate the itch intensity and impact of chronic pruritus on patients’ quality of life (QoL). These measures may be appropriate for evaluating the efficacy of anti-pruritic treatment in patients with chronic kidney disease-associated pruritus (CKD-aP). This study evaluated the psychometric validity and meaningful within-patient change (MWPC) thresholds of these two measures in patients with moderate-to-severe CKD-aP undergoing hemodialysis. Methodology Content validity interviews for the Skindex-10 were conducted in 23 patients. Psychometric properties of the Skindex-10 questionnaire and 5-D Itch scale were assessed using data collected from a phase 2 randomized controlled trial of an anti-pruritic treatment in patients with moderate-to-severe CKD-aP. As part of the trial, the patients (N = 174) had completed the Skindex-10 and 5-D Itch measures at baseline (Day 1, pre-treatment); Weeks 2, 4, 6; and end of treatment (Week 8). Anchor-based methods were used to determine MWPC score thresholds, which were verified in two very similar, larger phase 3 cohorts (N = 378 and N = 471). Results Content validity interview participants considered the Skindex-10 questionnaire to be straightforward, relevant, and comprehensive. Scores from both measures showed moderate-to-strong internal consistency (Cronbach’s α ≥ 0.74), and moderate test-retest reliability (intraclass correlation coefficients ≥ 0.67, Week 2 vs. Week 4) for patients stable on the similar Patient Global Impression of Worst Itch Severity measure. Construct validity analyses indicated that total scores and most domains of the Skindex-10 and 5-D Itch significantly correlated with other measures of itch. Skindex-10 and 5-D Itch total scores were also significantly different (P ≤ 0.0015) between distinct groups created based on conceptually related pruritus measures (known-groups validity). Anchor-based analyses indicated that a reduction from baseline of ≥ 15 points on the 0–60 scale for Skindex-10 total score, and of ≥ 5 points on the 5–25 scale for 5-D Itch total score, represented appropriate clinically meaningful within-patient changes. Conclusions The Skindex-10 and 5-D Itch showed good psychometric properties for measuring itch intensity and health-related quality of life in patients with moderate-to-severe CKD-aP. These results support their use in evaluating treatment efficacy in this patient population.