The development and content validation of an instrument to measure drainage in HS:The HISTORIC Drainage Scale

Background: Hidradenitis suppurativa (HS) is a chronic, debilitating inflammatory skin condition, often accompanied by malodorous drainage from HS lesions. Drainage significantly impacts patients' quality of life, causing social stigma and emotional distress. The HiSTORIC (Hidradenitis SuppuraTiva cORe outcomes set International Collaboration) initiative identified drainage as a critical symptom to be measured in clinical trials, yet no validated outcome measurement instrument (OMI) for HS drainage exists. This study aimed to develop and pilot-test a disease-specific patient-reported instrument to measure the amount of drainage in HS.<br/>Methods: An international workgroup, comprising researchers, clinicians, and patient research partners under the HiSTORIC initiative, was formed to guide development of the drainage instrument. The workgroup conducted a narrative review and designed candidate instruments based on insights from both existing wound drainage scales and HS-specific literature. Four versions of the scale were created and tested in a two-phase qualitative study, involving concept elicitation and pilot testing. Patients from dermatology clinics in Denmark and USA participated in cognitive interviews to assess the scales' content validity.<br/>Results: A total of 38 interviews (24 Danish, 14 U.S.) were conducted. Concept elicitation confirmed that no critical aspects of drainage were missing from the proposed scales. Pilot testing underscored the importance of measuring both "overall" and "worst" drainage levels, as these dimensions provided a more comprehensive understanding of patients' experiences. The final scale demonstrated comprehensibility, comprehensiveness, and relevance, and was iteratively refined based on patient feedback. The Numeric Rating Scale (NRS) was ultimately selected for its precision, feasibility, and alignment with existing HS measures.<br/>Conclusion: This study led to the development and content validation of a patient-reported outcome measure for drainage, the HISTORIC drainage scale for HS, that incorporates patient feedback to ensure relevance and usability. The final scale provides a standardized tool for assessing drainage in clinical trials, with potential for integration into routine care. Further validation is needed in the trial setting to investigate other aspects of validity and reliability.<br/>