Background A validated tool for the dynamic severity assessment of hidradenitis suppurativa/acne inversa (HS) is lacking. Objectives To develop and validate a novel dynamic scoring system to assess the severity of HS. Methods A Delphi voting procedure was conducted among the members of the European Hidradenitis Suppurativa Foundation (EHSF) to achieve consensus towards an initial HS Severity Score System (HS4). Strengths and weaknesses of HS4 were examined by a multicentre prospective study. Multivariate logistic regression, discriminant analysis and receiver operating characteristic curves, as well as examination for correlation (Spearman's rho) and agreement (Cohen's kappa) with existing scores, were engaged to recognize the variables for a new International HS4 (IHS4) that was established by a second Delphi round. Results Consensus HS4 was based on number of skin lesions, number of skin areas involved and Dermatology Life Quality Index (DLQI), and was evaluated by a sample of 236 patients from 11 centres. Subsequently, a multivariate regression model calculated adjusted odds ratios for several clinical signs. Nodules, abscesses and draining tunnels resulted as the scoring variables. Three candidate scores were presented to the second Delphi round. The resulting IHS4 score is arrived at by the number of nodules (multiplied by 1) plus the number of abscesses (multiplied by 2) plus the number of draining tunnels (multiplied by 4). A total score of 3 or less signifies mild, 4–10 signifies moderate and 11 or higher signifies severe disease. Cohen's kappa was fair (κ = 0·32) compared with Hurley classification, and moderate (κ = 0·49) compared with Expert Opinion. Correlation was good (ρ > 0·6) with Hurley classification, Expert Opinion, Physician's Global Assessment and Modified Sartorius score, and moderate for DLQI (ρ = 0·36). Conclusions The novel IHS4 is a validated tool to dynamically assess HS severity and can be used both in real‐life and the clinical trials setting.

Background: Validated, inclusive and easy‐to‐use outcomes for hidradenitis suppurativa are essential both in the clinical trial setting and clinical practice. The continuous IHS4 is a validated tool that dynamically assesses nodules/abscesses/draining tunnels and classifies disease severity as mild/moderate/severe. However, dichotomous outcomes are often required for clinical trials reporting.Objective: To develop and validate a dichotomous outcome based on IHS4 that can be used in clinical trial settings and day‐to‐day clinical practice.Methods: De‐identified data from the PIONEER‐I and ‐II studies were accessed through Vivli. Potential IHS4 thresholds were analysed using baseline to Week 12 data from adalimumab‐ and placebo‐treated hidradenitis suppurativa patients in the PIONEER‐I trial. The final threshold was chosen based on its ability to discriminate between patients treated with adalimumab or placebo and its association with reduction in inflammatory lesions. The final threshold was validated using data from baseline to Week 12 from adalimumab‐ and placebo‐treated hidradenitis suppurativa patients in both the PIONEER‐II and the combined PIONEER‐I and ‐II studies.Results: The best performing cut‐off for the IHS4 was a 55% reduction of the IHS4 score (IHS4‐55). Patients who achieved the IHS4‐55 had an odd's ratio of 2.00 [95%‐CI 1.26–3.18, p = 0.003], 2.79 (95%‐CI 1.76–4.43, p < 0.001) and 2.16 (95%‐CI 1.43–3.29, p < 0.001) for being treated with adalimumab rather than placebo in PIONEER‐I, PIONEER‐II and the combined dataset, respectively. Additionally, the achievement of the IHS4‐55 was associated with a significant reduction in inflammatory nodules, abscesses and draining tunnels in all analysed datasets.Conclusions: IHS4‐55, a novel dichotomous IHS4 version, based on a 55% reduction of the total score was developed. The IHS4‐55 performs similarly to the HiSCR in discriminating between adalimumab‐ and placebo‐treated hidradenitis suppurativa patients and shows significant associations with reductions in lesion counts. Moreover, the IHS4‐55 addresses some of the HiSCR drawbacks by dynamically including draining tunnels in a validated manner. By allowing the analysis of hidradenitis suppurativa patients with an abscess and nodule count below 3 but many draining tunnels, this outcome measure will improve inclusivity in clinical trials.