The prevalence of hidradenitis suppurativa outside the hospital setting: the impact of the undiagnosed.

Patients with hidradenitis suppurativa (HS) experience a significant delay from disease onset to diagnosis, estimated globally at 7·2 years. This is related to multiple factors including under‐recognition by doctors, embarrassment and stigma associated with the disease, and socioeconomic barriers. , , Global prevalence estimates vary widely, from as low as 0·1% up to 4% in one European study. , , This wide‐ranging prevalence estimate is likely to be due to the different methodologies employed in studies, including the use of population and healthcare databases and both validated and nonvalidated screening questionnaires with and without clinical diagnostic confirmation. The use of healthcare insurance databases probably underestimates the prevalence due to socioeconomic healthcare barriers and diagnostic delay affecting patients with HS, while nonvalidated screening questionnaires without clinical confirmation may overestimate the prevalence.

In this issue of the BJD, Prens et al. report the largest study thus far utilizing validated HS screening questions, in the prospective Lifelines Cohort Study in the Northern Netherlands. They received 58 198 out of a possible 135 950 responses. Patients in the study were asked if they had been diagnosed with HS. Those respondents who answered no were asked to complete two validated screening questions with a high sensitivity and specificity. Clinical photographs of HS lesions and the three Hurley stages were provided as a diagnostic aid for respondents.

Overall, 448 respondents reported a previous diagnosis of HS and 708 respondents answered positively to the two screening questions (1156 total prevalent cases of HS). The overall prevalence was thus 2·1%. Only 49 participants were receiving treatment for their HS (4·2%), with 30 of those receiving treatment under the care of a dermatologist. If the authors had utilized the data on medically diagnosed HS alone to estimate prevalence it would be much lower at 0·8%, highlighting the gap in prevalence, which may be related to the lengthy diagnostic delay.

Comorbidity analysis in this large cohort confirmed the association of HS with significant comorbidities including obesity, type 2 diabetes, chronic obstructive pulmonary disease, depression and Crohn disease. There were several newly identified comorbidities including fibromyalgia, irritable bowel syndrome, chronic fatigue syndrome and migraine.

A previous systematic review and network meta‐regression analysis identified an overall prevalence of 0·4% in studies from Europe, the USA and Australia. This identified the disparity between clinical samples in healthcare settings, with a pooled prevalence of 1·7%, and population‐based studies including the use of healthcare databases and screening questionnaires, which had a much lower pooled prevalence of 0·3%. Prens et al. demonstrate this in a large population‐based study, with a gap in prevalence rates between patients at inclusion in the cohort study (0·8%) and patients in the final analysis (2·1%). The strengths of this study are in the use of validated screening questions and the large sample size representative of an entire population and not just patients within a hospital setting. The prevalence rate of 2·1% seen in this study is likely to be most representative of the actual prevalence of HS, and once again highlights the diagnostic delay and the need for increased awareness of HS among physicians and patients.