INTRODUCTION: Itch associated with atopic dermatitis (AD) has a profoundly negative effect on patients of all ages. Therefore, itch is a main target for AD therapeutic approaches, and treatments are perceived as beneficial when they achieve an itch reduction. In the absence of a validated scale for children aged 6–11 years that is suitable for assessing itch intensity in clinical trial settings, the Worst Itch Scale was developed. METHODS: Qualitative interviews, comprising concept elicitation and cognitive debriefing, were conducted to develop and evaluate the content validity of the Worst Itch Scale. Psychometric assessments used data from the LIBERTY AD PEDS phase 3 trial of dupilumab in patients aged 6–11 years with severe AD. These included test–retest reliability, construct validity, known-groups validity and responsiveness. Thresholds for clinically meaningful change were defined using anchor- and distribution-based methods. RESULTS: The Worst Itch Scale consisted of two items asking about “worst itching” experienced “last night” and “today”. Worst Itch Scale scores showed large, positive correlations with existing patient-reported outcome (PRO) measures of itch and weaker correlations with clinician-reported outcome (ClinRO) measures assessing objective signs of AD. Improvements in Worst Itch Scale scores were highly correlated with improvements in other itch PROs and moderately correlated with improvements in ClinROs. The responder definition based on the primary anchor, a 1-point improvement in the Patient Global Impression of Disease, was 2.84. Supportive anchors produced response estimates ranging from 2.43 to 4.80 points. CONCLUSIONS: The Worst Itch Scale is a fit-for-purpose (e.g. well-defined, reliable, responsive and valid) scale for evaluating worst itch intensity in children aged 6–11 years with severe AD. The within-patient threshold for defining a clinically meaningful response was a ≥ 3–4-point change in the Worst Itch Scale score.
