Background: SSIs are common and preventable adverse surgical outcomes that account for substantial morbidity and mortality. Pre-operative antimicrobial administration reduces SSI; post-operative antimicrobials do not reduce SSI but do increase other post-operative adverse events. In 2005, the VA implemented the Surgical Care Improvement Project (SCIP) to increase compliance with peri-operative quality metrics, including appropriate antimicrobial use. SCIP included a set of publicly reported evidenced-based antimicrobial guideline compliance metrics targeting high-risk surgeries. The metrics required resource-intense manual review as part of the VA’s External Peer Review Program (EPRP). In 2015, SCIP achieved sustained compliance with antimicrobial use metrics exceeding 95% and the program was retired due to the high costs of manual review with minimal expected additional benefit. This IIR proposal is guided by the Dynamic Sustainability Framework and examines whether the antimicrobial practice changes achieved by SCIP were sustained and how they spread to surgeries and practice areas beyond the original scope of the program. This multiple PI submission by Dr. Hillary Mull, PhD and Dr. Westyn Branch-Elliman, MD, MMSc builds and expands upon their prior VA-funded collaborative work applying electronic algorithms to measure antimicrobial use and identify adverse outcomes. Significance/Impact: Improving antimicrobial prescribing and surgical quality are major VA goals. In FY 2018, within the VA, there were 190,000 surgeries among SCIP-targeted specialties (cardiac, orthopedic, general/colorectal, gynecology and vascular). Of these, 64% were outpatient, where the PIs found high rates guideline discordant post-operative antimicrobial use, which contribute to substantial patient harms. Thus, the potential impact of the research on direct clinical care is high. In addition, there are limited studies examining what happens to practice change after discontinuation of active programs designed to support compliance with evidence-based guidelines. The proposed study will close this gap in implementation science research. Innovation: In line with the VA’s Learning Health System directive, this project will use the VA’s electronic health record data to develop algorithms to identify pre-and post-operative antimicrobial use based on the PI’s prior work. These electronic data mining algorithms capture structured and text note data and represent a significant technological advancement over costly manual review. This study also answers important questions about the sustainability of practice change after discontinuation of an active policy with input from frontline staff. Specific Aims: 1) Measure sustainability of antimicrobial prophylaxis guideline compliance after SCIP retirement and assess whether practice changes spread to SCIP-excluded procedures; 2) Assess facilitators and barriers to implementation sustainability through staff interviews, and map results...