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

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. 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). 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. 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. Patients with advanced chronic kidney disease often suffer from intense and bothersome itching, which can greatly lower their quality of life. To determine whether new treatments for itching are effective, clinicians need to assess if the itching and its impact on patients’ lives improve over time. Only the patients themselves can accurately report how severe the itching is and how it affects their daily lives. Patients can fill out questionnaires and scales to provide this information, but these tools need to be tested to ensure they work well for this specific group of patients. This study tested two tools for measuring how itching affects the quality of life in patients with advanced chronic kidney disease. These tools are called the “Skindex-10 questionnaire” and the “5-D Itch scale”. Using data from previous clinical trials of an anti-itch treatment in these patients, we found that both tools gave reliable results in test-retest experiments, and the results matched well with other established methods for measuring itch impact. For each measure, we also calculated how much change represents a meaningful improvement for patients with chronic kidney disease who had moderate-to-severe itching at the start of treatment. These findings support the use of these two tools to assess whether treatments for itching truly help patients with chronic kidney disease.