Abstract Introduction In appendicitis, antibiotics are routinely commenced prior to consideration of operative management. The choice of initial regimen depends on local antimicrobial guidelines. The presence of resistant organisms risks development of post-operative collections. In our tertiary paediatric surgery unit, antimicrobial guidelines were changed in September 2022 to recommend co-amoxiclav instead of cefuroxime/metronidazole. This study compares incidence of post-operative collections before and after the policy change. Method Patients who underwent appendicectomy between September 2021 and October 2023 were identified and contemporaneous data collected. Patients who had initial surgery elsewhere, interval/incidental appendicectomies or penicillin allergy were excluded. Comparison was made between patients receiving cefuroxime/metronidazole (Group 1) or co-amoxiclav (Group 2). Results Over the 26-month period, there were 104 patients in Group 1 (after excluding 47) and 104 in Group 2 (after excluding 46). 4 (3.8%) developed collections in Group 1 versus 13 (12.5%) in Group 2. In Group 1, 2 (50%) were treated with antibiotics alone, 1 (25%) required drain insertion and 1 (25%) had a laparotomy. In Group 2, 7 (54%) were treated with antibiotics alone, 5 (38%) required drain insertion and 1 (8%) had a laparotomy. The relative risk of developing a post-operative collection was 3.3 (p=0.023) in Group 2 versus Group 1. Conclusions The change in antimicrobial policy resulted in a significant increase in post-operative collections in our unit. This provides evidence that a regimen of cefuroxime/metronidazole is more effective at preventing collections and we are liaising with our Microbiology department with a view to amend the guidelines.