Qualitative analysis and reproducibility assessment of the Scalp Itch Numeric Rating Scale among patients with moderate to severe plaque psoriasis of the scalp.

Background: Pruritus is a prevalent and bothersome symptom of scalp psoriasis. Validated scales assessing scalp itch are needed to evaluate treatment efficacy. Objective: To evaluate comprehensibility and reproducibility of the Scalp Itch Numeric Rating Scale (NRS), a novel scale being used in a phase 3 study of apremilast. Methods: The Scalp Itch NRS, Modified Whole Body Itch NRS, Global Assessment of Psoriasis Severity-Scalp (GAPS-S), and Global Impression of Change-Scalp Itch (GIC-SI) were assessed among patients with moderate to severe scalp psoriasis. Convergent validity and test-retest reliability between two visits (7 ± 3 days apart) were assessed using intra-class and Spearman's correlations. Results: Patients found the Scalp Itch NRS easy to use and understand. Convergent validity (Modified Whole Body Itch NRS Visit 1: rs  = 0.71, Visit 2: rs  = 0.92, p < .0001; GAPS-S Visit 1: rs  = 0.62, Visit 2: rs  = 0.63, p < .0001), and consistency with changes (Modified Whole Body Itch NRS: rs  = 0.69, p < .0001; GAPS-S: rs  = 0.42, p  = .0029) were demonstrated. The Scalp Itch NRS showed strong test-retest reliability (intra-class correlation coefficient = 0.87; rs  = 0.89). Change scores on the Scalp Itch NRS were consistent with change scores on the GIC-SI. Conclusions: The Scalp Itch NRS is a valid and reproducible measure of scalp itch in patients with moderate to severe scalp psoriasis. ClinicalTrials.gov: NCT03123471.