External validation of the IHS4-55 in a European antibiotic-treated HS cohort

Background: Previously, a new dichotomous outcome was developed, calculated as 55% reduction in the International Hidradenitis Suppurativa 4 score (IHS4-55). It was validated in datasets of adalimumab and placebo treated HS patients. External validation is an important aspect of clinical outcomes. Objectives: We aimed to externally validate the novel dichotomous IHS4-55 in a non-biologic treated dataset of HS patients. Methods: Data from a previously published European-wide prospective clinical study of antibiotic treatment of HS patients was used to assess the association of IHS4-55 achievement with individual reduction in inflammatory nodules, abscesses and draining tunnels. Moreover, the associations between IHS4-55 positivity and achievement of the minimal clinically important differences (MCID) for Dermatology Life Quality Index (DLQI), numerical rating scale (NRS) Pain, and NRS Pruritus were analyzed. Results: Data was obtained from 283 individual patients, of which 36.4% (103/283) were treated with clindamycin and rifampicin and 63.6% (180/283) with tetracyclines for 12 weeks. Achievers of the IHS4-55 demonstrated a significant reduction the counts of inflammatory nodules, abscesses, and draining tunnels (all p<0.001). Additionally, IHS4-55 achievers had an odds ratio (OR) for achieving the minimal clinically important difference (MCID) of DLQI, NRS Pain and NRS Pruritus of 2.16 (95% CI 1.28-3.65, p<0.01), 1.79 (95% CI 1.10-2.91, p<0.05), and 1.95 (95% CI 1.18-3.22, p<0.01), respectively. Conclusions: This study shows the external validity of the novel IHS4-55 by demonstrating a significant association between IHS4-55 achievement and a reduction in inflammatory lesion counts as well as achievement of MCIDs for DLQI, NRS Pain and NRS Pruritus in an antibiotic-treated cohort. These findings support the use of the IHS4-55 as a novel primary outcome measure in clinical trials.