Supplementary Material for: Prevalence of Hidradenitis Suppurativa in Lyon: results from the French team of the Global Hidradenitis Suppurativa Atlas project

Introduction: The exact prevalence of hidradenitis suppurativa (HS) remains elusive, in France as elsewhere. The GHiSA (Global Hidradenitis Suppurativa Atlas) project is a unique opportunity not only to determine the exact prevalence in each participating centre, but also to help estimate the global prevalence and provide a basis for comparing centres with each other and obtaining potential pathophysiological indications. Methods: A French version of the questionnaire developed for the GHiSA initiative was proposed to healthy adults accompanying a patient to the outpatient clinic. Results: The prevalence of HS in the 525 participants was 3.4% (95% confidence interval: 2.2% – 5.4%). The HS patients were 10 females and 8 males, were significantly younger than non-HS controls and more frequently smokers. The screening questionnaire showed a 100% sensitivity and negative predictive value. Discussion: The estimated prevalence here is more than three times higher than the 1% prevalence usually recognised in Western countries. It remains to be determined whether this increase is apparent (freedom of speech; methodological differences compared with previous studies, which contribute to the usual figure of 1%) or real. In the latter case (gross increase in the prevalence of the disease), the underlying mechanisms need to be explored, and comparison with the results observed in other GHiSA centers will be decisive.