Background: Hidradenitis suppurativa (HS) is a chronic, inflammatory skin disease with a large impact on patients’ health-related quality of life. However, reliable and consistent outcome measures to assess body surface area (BSA) of HS have not been established. Objectives: To develop and assess the reliability and validity of a novel outcome instrument for assessment of HS BSA in a clinical trial setting. Methods: Qualitative interviews and focus groups were conducted from July to August 2015 and October 2017 to January 2018. Evaluation of the measurement was assessed during a single-day grading session with patients in April 2018. Participants, who included clinicians or patients, were recruited from academic medical centres in the U.S. mid-Atlantic region. Results: Concept elicitation included input from 10 providers, of which 60% (n = 6) were female, 80% (n = 8) dermatology specialists and 20% (n = 2) gynaecology specialists. Cognitive debriefing was conducted with 11 providers, of which 82% (n = 9) were dermatologists and 18% (n = 2) gynaecologists. The evaluation stage included 10 clinicians and 23 patients. The intraclass correlation coefficient (ICC) for inter-rater reliability was 0·60 [95% confidence interval (CI) 0·44–0·74]. The ICC for intrarater reliability was 0·98 (95% CI 0·94–1·00). Transformation of the BSA score resulted in an increase in inter-rater reliability to 0·75 (95% CI 0·62–0·85) or 0·76 (95% CI 0·62–0·85). Scores all demonstrated concurrent validity, with statistically significant correlations with extant scoring methods. Conclusions: This novel scale is a reliable and valid HS outcome instrument and may capture a wide range of patients by assessing BSA. Future research is necessary to demonstrate its responsiveness. What's already known about this topic?. The major HS disease activity scales rely on lesions counts and have moderate-to-good reliability. Body surface area (BSA) is one of the physical signs included in the Core Outcome Set for HS, but is not a part of existing HS disease activity scales. What does this study add?. A novel disease severity scale, the Severity and Area Score for Hidradenitis (SASH), was developed and the psychometric properties assessed. There was high inter-rater reliability of 0·75 and 0·76 when BSA was scored on an ordinal scale, and an excellent intrarater reliability of 0·98. The SASH score also demonstrated convergent validity with extant instruments. What are the clinical implications of this work?. The ability of clinicians to accurately assess disease status will be improved. Implementation of the SASH score will help guide and assess the effectiveness of appropriate treatment choice.
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