Incidence of Dermatitis Herpetiformis in Sweden 2005 to 2018: A Nationwide Retrospective Cohort Study

Dermatitis herpetiformis has been investigated in the past; however, only a limited number of studies have reported its incidence based on validated nationwide population-based registries. To address this gap, the aims of this study are to estimate the incidence of dermatitis herpetiformis in Sweden and to validate the National Patient Register (NPR) for diagnosis of dermatitis herpetiformis. A population-based open cohort study was conducted, including all patients diagnosed with dermatitis herpetiformis (International Classification of Diseases 10th revision; ICD-10 code L13.0) in Sweden from 2005 to 2018 (n = 1,724), identified from the NPR. The diagnosis of dermatitis herpetiformis in the NPR was validated using medical records, histopathological and immunopathological data, yielding a positive predictive value (PPV) of 62.5%. The mean annual incidence of dermatitis herpetiformis was 0.93/100,000 (95% confidence interval 0.79–1.08), female to male ratio 1:1, and mean age at diagnosis 60.9 years. In conclusion, this large nationwide cohort study showed a low validity for diagnosis of dermatitis herpetiformis in the NPR, and the adjusted incidence rate of dermatitis herpetiformis in Sweden was estimated to be 0.93/100,000, which is lower than that in previous Swedish studies. SIGNIFICANCE This study reveals important findings regarding the incidence of dermatitis herpetiformis in Sweden as well as the accuracy of the Swedish National Patient Register. The incidence of dermatitis herpetiformis in Sweden is 0.93 per 100,000 inhabitants and the study indicates that dermatitis herpetiformis is more common among the elderly population. The incidence rate for women was significantly higher in the age range 30–49 years, while men had a significantly higher incidence rate for those aged 60 years or older. Information on dermatitis herpetiformis in the National Patient Register is not comprehensive, and miscoding of International Classification of Diseases 10th revision (ICD-10) codes has to be taken into account.