Title: Surgical Site Infection in Elective Colonic & Rectal Resections: Effect of Oral Antibiotics & Mechanical Bowel Preparation vs. Mechanical Bowel Preparation Only

AIM: Surgical site infections are disproportionately common after colorectal surgery and may be largely preventable. The objective of this retrospective cohort study was to determine the effect of oral antibiotics with mechanical bowel preparation on surgical site infections. METHOD: A retrospective study of a consecutive series of elective colonic and rectal resections following an Enhanced Recovery After Surgery pathway, which also included mechanical bowel preparation from September 2014 to September 2017. The addition of oral antibiotics, neomycin and metronidazole, with mechanical bowel preparation was assessed. Development of surgical site infections within 30 days was the main outcome of measure. The secondary outcome was assessment of possible surgical site infection predictors. RESULTS: 732 patients were included: 313 (43%) pre-intervention and 419 (57%) post-intervention. Surgical site infection rates pre vs. post intervention were: overall 20.8% vs. 10.5%, p<0.001; superficial 10.9% vs. 4.3%, p<0.001 and organ space 9.9% vs. 6.2%, p=0.03. Subgroup analysis of colonic resections revealed a significant reduction in overall (17.1% vs. 6.8%), superficial (10.7% vs. 4.3%) and organ space infections (6.4% vs. 2.6%). Rectal resections had significant reduction in overall (26.2% vs. 15.3%) and superficial infection rates (11.1% vs. 4.4%), but not organ space infections (15.1% vs. 10.9%). Multivariate regression analysis revealed open vs. minimally invasive surgery (MIS) (p<0.001) and omission of oral antibiotics (p=0.004) as independent predictors of surgical site infections. CONCLUSION: A significant reduction in superficial and organ space infections was found after colonic resections with use of oral antibiotics; however, a significant reduction was found in only superficial infections following rectal resections with oral antibiotics.