Su1465 The Prevalence of Sessile Serrated Adenomas in a Fecal Immunochemical Test Positive Colorectal Cancer Screening Cohort Nicholas J. Tutticci*, Barbara A. Leggett, Mark N. Appleyard, David G. Hewett Gastroenterology and Hepatology, Royal Brisbane and Womens Hospital, Brisbane, QLD, Australia; School of Medicine, University of Queensland, Brisbane, QLD, Australia Background: Sessile serrated adenomas (SSAs) are important, colonoscopicallysubtle precursor lesions in the serrated pathway of colorectal neoplasia. SSA prevalence rates have been described in cohorts undergoing colonoscopy for average risk screening and for clinical indications, but not in a high risk screening cohort (fecal immunochemical test positive, FIT ). We aimed to assess the prevalence of SSAs in participants from the Australian Bowel Cancer Screening Program, a national, population-based screening program offering fecal immunochemical testing to adults aged 50, 55 and 65, and colonoscopy for those returning a positive result. Methods: We enrolled 503 consecutive FIT screening program participants from a defined geographic catchment between 2008 and 2011. We assembled a comparison, non-screening control cohort of 1056 patients 50 years of age undergoing colonoscopy by matched colonoscopists for clinical indications from the same center. We used multiple logistic regression to analyze the predictors of SSAs in FIT screening participants, and compare the prevalence of SSAs between screening participants and controls, adjusting for age, sex, bowel preparation, colonoscopist and year of colonoscopy. Results: 164 SSAs were found in 64 (13%) of 503 FIT participants, representing 18% of all polyps; 114 (70%) of SSAs were proximal. Adenomas were found in 259 (52%) FIT screening program participants. Of these, 129 (26%) had advanced adenomas and 46 (5%) had synchronous SSAs. 10 cancers were found, none with synchronous SSAs. Among FIT screening participants, the presence of an SSA was significantly associated with the presence of synchronous adenomas (OR 2.67, p 0.002) and colonoscopist (OR 3.87, p 0.002) but not participant age, sex, year of procedure or bowel preparation. In the control group, adenomas were found in 284 (27%) patients, advanced adenomas in 80 (8%) and SSAs in 61 (6%). The crude prevalence of SSAs was 13% in FIT screening participants, compared with 6% in control patients. This difference was significant (adjusted OR 1.9, p 0.01) after controlling for age, sex, bowel preparation, colonoscopist and year, but not when controlling for the presence of an adenoma (adjusted OR 1.43, p 0.157). Conclusions: FIT screening program participants undergoing colonoscopy have a higher prevalence of SSAs than patients undergoing colonoscopy for clinical indications. However, this difference likely mirrors the higher prevalence of adenomas also seen in this screening cohort, rather than the detection of SSAs by FIT. Further, any differences need to be interpreted with caution given the variation in SSA prevalence between colonoscopists.