<h3>Background:</h3> Eosinophilic granulomatosis with polyangiitis (EGPA) is an ANCA-associated vasculitis (AAV) characterized by hypereosinophilic asthma with vasculitis manifestations. The incidence of EGPA ranges between 0.5 and 4.2 cases per million inhabitants per year and its prevalence is between 2 and 30.4 cases per million inhabitants. Further, an increasing annual incidence rate of overall AAV has been reported. No data about the epidemiology of EGPA in Italy are available. <h3>Objectives:</h3> To estimate the prevalence and the incidence of EGPA in a region of Northern Italy over 32 years (from January 1991 to December 2022) and the trends in incidence rates in the last two decades. <h3>Methods:</h3> This population-based study was conducted in a region of Northern Italy with 529,000 inhabitants. The observation period went from January 1991 to December 2022. 6 sources were used to ascertain cases: the outpatient clinical records of the vasculitis and asthma clinics, the database of the centralized laboratory searching for positive ANCA results, the drug prescription database searching for patients receiving IL5 inhibitors, the hospital discharge database (ICD-9 codes), and the centralized registry for rare diseases, which was created to identify all patients with rare diseases, including EGPA, to exempt patients from payment of disease-related medical costs. Two clinicians reviewed all the identified cases. Only patients with a final diagnosis of EGPA satisfying the 2022 ACR/EULAR classification criteria were included. A capture-recapture model was built to estimate EGPA prevalence at 1/1/2023 and incidence in the study period. Based on the overlap of different sets, the analysis permits the estimate of the cases not captured by any source. The best estimate was obtained by selecting the log-linear model providing the best compromise between the least interaction terms and the highest adequacy as measured by the likelihood ratio statistics (Akaike's information criterion [AIC], Bayesian information criterion [BIC]). To assess whether incidence rates increased over time, capture-recapture analyses were also conducted in two periods (2001-2011; 2012-2022). Cases diagnosed before 2001 were excluded due to the unavailability of the sources. <h3>Results:</h3> 40 cases with a final diagnosis of EGPA satisfying the 2022 ACR/EULAR classification criteria were identified in the study period and were included. Clinical features are reported in Table 1. The mean age at diagnosis was 55 years. 4 patients deceased during the study period. Among the 36 prevalent cases, 8 (22.2%) were derived from a single source, 7 (19.4%) from two, 9 (25%) from three, 10 (27.8%) from four and 2 (5.6%) from five sources. The best fitting capture-recapture model included only the interaction between the drug and ANCA databases, and the discharge diagnosis codes and rare diseases registry (Figure 1). According to the model, the prevalence at January 1<sup>st</sup>, 2023 was 68 cases per million inhabitants, and 0 missed cases (95%CI 0-4, AIC -2.46, BIC -2.35). The overall crude annual incidence was 3.1 per million inhabitants. 15 and 22 cases occurred in the 2001-2011 and 2012-2022 periods respectively; capture-recapture models estimated 0 missed cases (95% CI 0-3) in both periods. Age and sex standardized incidence rates were 2.83 (95% CI 1.39-4.27) and 3.59 (95% CI 2.08-5.09) per million inhabitants per year, respectively. <h3>Conclusion:</h3> EGPA is a rare disease. In line with other studies, we observed a significant increase in the incidence of the disease in the last 20 years. Capture-recapture analysis suggests a small, if any, loss of cases, indicating that routinely collected data sources allow to accurately identify incident and prevalent cases.<b>REFERENCES: NIL.</b> <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>

AB1273 EPIDEMIOLOGY OF EOSINOPHILIC GRANULOMATOSIS WITH POLYANGIITIS IN NORTHERN ITALY

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