P-29. Impact of Reclassification of Serratia marcescens as a Low-Risk AmpC Pathogen on Treatment Selection and Outcomes in Bacteremia

Abstract Background In 2021, IDSA reclassified S. marcescens as a low-risk organism for clinically significant AmpC production, expanding treatment options to include ceftriaxone and piperacillin-tazobactam. In July 2021, the health system laboratory adopted the recommended reclassification, removed historic interpretive comments from the susceptibility report, and implemented selective reporting for cefepime and carbapenem susceptibilities. The objective of this study was to determine the impact of reclassification of S. marcescens as a low-risk AmpC pathogen on antibiotic selection and clinical outcomes. Methods This was a retrospective chart review of adult inpatients with S. marcescens bacteremia susceptible to ceftriaxone or piperacillin-tazobactam during the pre-reclassification (preRC; July 2018 – June 2021) and post-reclassification (postRC; July 2021 – December 2024) study periods. Patients were excluded if they were deceased or made comfort care prior to receiving definitive antibiotic therapy, discharged prior to susceptibility results, transferred from an outside hospital, neutropenic, had a presumed or documented concomitant infection or polymicrobial infection. The primary endpoint was the difference in percentage of patients treated with cefepime or carbapenems in both groups. Secondary endpoints were evaluated in the subgroup receiving ≥5 days of definitive monotherapy with ceftriaxone or piperacillin-tazobactam (CPT) or cefepime or a carbapenem (CC) within 48 hours of susceptibility results, and compared clinical outcomes based on treatment selection. Results A total of 87 patients were included. An absolute reduction of 34.1% in treatment with cefepime or carbapenems was noted in the postRC group (54.5% vs 20.4%, p = 0.001). Secondary endpoints were analyzed in 57 patients. No statistically significant differences were noted in clinical failure, mortality, length of stay or readmission within 30 days. A significant reduction in median carbapenem days of therapy was observed in the postRC group (6 vs. 0, p