Regional variation of hidradenitis suppurativa in the Norwegian Patient Registry during a 5‐year period may describe professional awareness of the disease, not changes in prevalence

Dear Editor, Hidradenitis suppurativa (HS) is a distinct skin disease characterized by well‐defined signs, symptoms and a notorious tendency to recurrence and chronicity.1 Nevertheless, considerable uncertainty exists regarding its prevalence. The prevalence has been estimated over recent decades with a significant degree of unexplained discrepancy, ranging from 0·05% to 8% in available studies.2 In general, registry‐based studies tend to indicate lower prevalence rates than reports based on physician‐examined or self‐reported data. It may be suspected that registry data are influenced by local variations in the healthcare system. We accessed the Norwegian Patient Registry, which is a public registry providing basic descriptive data on all patients treated in public hospitals, policlinics and by practising specialists under public contract. The registry is based on International Classification of Diseases (ICD)‐10 coding and allows for the comparison of registry‐based prevalence estimates of different diseases for the different health regions of Norway. Therefore, it offers data generally comparable with data described in other registry studies. We investigated possible differences in the number of patients treated for HS in various Norwegian health regions in order to explore the variation in prevalence rates. Patients were identified based on the ICD code for HS (L73.2) and simple descriptive statistics provided for each region. Data are shown in Figures 1 and 2. Rate 1/10 000 diagnosed as L73.2 (International Classification of Diseases 10 coding) in 2002 (blue) and 2006 (red) for the different health authorities of Norway. Patients diagnosed as L73.2 per year in the period 2002–2006 in Norway. There was an increase in the overall number of patients (white bars, men; light grey bars, women; dark grey bars, total) registered from 1131 patients to 1677 patients. Two relevant observations could be made based on the data. Firstly, the prevalence rates varied widely. The number of cases coded by specialists show large differences between the five different health regions of Norway (Fig. 1). In Health Region 5 in northern Norway (Troms and Finnmark) twice as many patients with HS were identified compared with the other four more populous regions (Fig. 1). Secondly, in agreement with U.S. data,3 an increased number of patients were added to the registry at an accelerated rate during this period (Fig. 2). The Norwegian data suggests that prevalence estimates based on public or hospital data may vary greatly even in small populations with easy access to healthcare services.4 Therefore, it is suggested that such prevalence estimates are likely to be biased by awareness and intention to treat. Epidemiological data based on patients treated must therefore be considered as an unreliable source of actual epidemiological data for HS in the population. However, it can be used to estimate how often a particular disease is recognized and treated, and the Norwegian data indicates that a considerable unmet need for treatment exists for HS. Funding sources: none. Conflicts of interest: none declared.