Importance: Standard morphological terminology and definitions are vital for identification of lesion types in the clinical trial setting and communication about the condition. For hidradenitis suppurativa (HS), morphological definitions have been proposed by different groups, representing various regions of the world, but no international consensus has been reached regarding such definitions. A lack of globally harmonized terminology and definitions may contribute to poor-quality data collection in clinical trials as well as poor communication among clinicians, investigators, and patients. Objective: To establish and validate consensus definitions for typical morphological findings for HS lesions. Methods: This study was conducted from August 2019 to August 2020. A Delphi study technique was used to assess agreement and then resolve disagreement on HS terminology among international experts. After an initial preparation phase, the process consisted of 3 rounds. In each round, participants reviewed preliminary definitions and rated them as "keep, with no changes," "keep, with changes," or "remove." Eight HS primary lesions, including papule, pustule, nodule, plaque, ulcer, abscess, comedo, and tunnel, were selected because they are most frequently used in HS clinician-reported outcome measures. The initial definitions were based on extant literature, and modifications were made between rounds based on qualitative thematic analysis of open-ended responses or discussion. Consensus was defined as greater than 70% to accept a definition. Consensus stability across rounds was defined as less than 15% change in agreement. Reliability was evaluated using the Gwet agreement coefficient. Validation was based on assessment of face validity and stability across rounds. Results: A total of 31 experts participated. All 8 HS primary lesion definitions achieved greater than 70% consensus by Delphi round 3. Stability was achieved for papule, pustule, and abscess. The Gwet agreement coefficient increased from 0.49 (95% CI, 0.26-0.71) in round 1 to 0.78 (95% CI, 0.64-0.92) in round 3. Face validity was supported by expert endorsement to keep terms in survey responses. Previously unmeasured variation among clinicians' definition of tunnels was identified, and consensus was achieved. Conclusions and Relevance: An international group of experts agreed on definitions for morphological features of HS lesions frequently included in HS clinical trials. These international consensus terms and definitions are needed to support consistency of lesion identification and quantification in clinical trials..
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