SummaryThe recent hidradenitis suppurativa (HS) Cochrane review identified outcome measure heterogeneity as an important issue to address when designing future HS trials. Our objective was to follow the Harmonising Outcome Measures for Eczema (HOME) roadmap, by performing a systematic review of HS outcome measure instruments to inform the development of an HS core outcome set. We performed a systematic review to identify validation evidence for outcome measure instruments used in HS randomized controlled trials (RCTs), and assessed the methodological quality of all HS outcome measure validity studies using the COnsensusbased Standards for the selection of health Measurement INstruments (COSMIN) checklist. The 12 RCTs included in the Cochrane review utilized 30 outcome measure instruments, including 16 physician-reported instruments, 11 patientreported instruments and three composite measures containing elements of both. Twenty-seven (90%) of the instruments lacked any validation data. Two further instruments have been developed and partially validated. Of the seven studies meeting our inclusion criteria, six were of 'fair' or 'poor' methodological quality, in part because most of the studies were not primarily designed for instrument validation. The HiSCR instrument is supported by good-quality validation data, but there are gaps, including assessment of internal consistency, inter-rater reliability and minimal clinically important difference, and convergent validity fell below the acceptable range for some comparisons. Multiple, usually unvalidated, outcome measure instruments have been used in HS RCTs. Where validation evidence is available there are issues of low methodological quality or incomplete validity assessment and so, currently, no instruments can be fully recommended.What's already known about this topic?• The recent hidradenitis suppurativa (HS) Cochrane review identified heterogeneity of outcome measure instruments as an important obstacle in the design of future HS trials.• The Harmonising Outcome Measures for Eczema (HOME) initiative provides a roadmap for developing a core outcomes set in HS.What does this study add?• Twenty-seven of the 30 outcome measure instruments used in HS randomized controlled trials are not supported by any formal validation data.• Where available, validation evidence is generally of relatively low methodological quality, or remains incomplete, and so no instruments can be fully recommended currently.

No abstract