Validation of the Peak Pruritus Numerical Rating Scale as a Patient-Reported Outcome Measure in Prurigo Nodularis

IntroductionValidated patient report tools for quantifying patient experiences of itch in prurigo nodularis (PN) are limited. This study aimed to evaluate the validity of the 11-point peak pruritus numerical rating scale (PP NRS) as a single-item patient-reported outcome (PRO) measure for assessing itch severity in PN.MethodsContent validity of the PP NRS was evaluated through qualitative interviews with adults with PN. The PP NRS was then psychometrically evaluated using data from a placebo-controlled trial of nemolizumab in adults with PN, during which patients completed the PP NRS daily. Meaningful within-patient change was estimated from the qualitative interviews and by anchor- and distribution-based analyses of trial data.ResultsThe interview participants (N = 21) all understood the PP NRS and reported itch as their worst symptom overall. The PP NRS showed good test-retest reliability and demonstrated convergent validity and known-groups validity. PP NRS scores improved more in patients classified as "improved" on other clinical outcome measures than in those classified as "worsened/unchanged". Triangulation of the different estimates identified a 2- to 5-point decrease in PP NRS score as a meaningful within-patient change threshold.ConclusionThe PP NRS is a content-valid and reliable PRO measure for quantifying itch severity in adults with PN in clinical trials.Trial Registration NumberNCT03181503. Prurigo nodularis is a skin disease where the skin becomes inflamed and very itchy. Itching and scratching can ruin the lives of people with prurigo nodularis. For doctors to know how well treatments for prurigo nodularis are working, they need to be able measure how bad their patients' itching is. Here we tested whether a tool called the peak pruritus numeric rating scale (PP NRS) can be used to reliably measure itching in patients with prurigo nodularis. The PP NRS asks patients to rate their itch "at the worst moment during the previous 24 h" on a scale from 0 ("no itch") to 10 ("worst itch imaginable"). We interviewed 21 patients with prurigo nodularis. Overall, itch was their worst symptom. They were able to use the PP NRS with few or no problems. To further explore whether the PP NRS can be used by patients with prurigo nodularis, we also obtained data from a clinical study. In this clinical study, patients with prurigo nodularis received a drug that might one day be used to treat their disease. They completed the PP NRS daily during the time they received the drug. When we analyzed the data from the clinical study, we found that the PP NRS has the properties a tool like this needs to be useful. We also concluded that a decrease of 2-5 points on the PP NRS scale would be a meaningful improvement in itching for patients with prurigo nodularis.