Abstracts of the 12 th Congress of ECCO -European Crohn's and Colitis Organisation S359surgery trigger alteration of the intestinal microbiome in predisposed patients. This may result in chronic inflammation of the bowel mucosa causing changes seen in Crohn's disease. Background: Reactivation of hepatitis B virus (HBV) is one of the most important side effects in IBD patients with HBV infection treated with anti-TNF-alpha agents. We investigated the rate of prophylaxis and the risk of HBV reactivation in HBV infected patients with IBD receiving anti-TNF-alpha therapy.Methods: This was a retrospective multicenter study including 14 academic teaching hospitals in Korea. IBD patients with HBV infection (HBsAg-positive) who treated with anti-TNF-alpha agents were enrolled. Medical records of patients were reviewed and data were collected using web-based case report form.Results: A total of 61 patients (18 UC, 43 CD) were included. 70% were male and mean age at diagnosis was 34.9±12.0 years. Indications for anti-TNF-alpha therapy were steroid-dependency, refractoriness to conventional therapies, or fistulizing disease. Among 43 patients who tested serum HBV-DNA levels, 35 (81%) were positive for HBV-DNA prior to anti-TNF-alpha therapy. Only half of the patients (51%) received prophylactic anti-viral agents. During the follow-up of median 24 months, 16.4% of patients experienced

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