Optimal Spectrum Antibiotic Ratio for assessing antibiotic utilisation in sepsis caused by gram-negative bacilli

BACKGROUND: Accurate monitoring of antimicrobial consumption is essential for developing effective stewardship interventions. Hospitals are mandated to report antibiotic treatment duration using the CDC's Standardized Antimicrobial Administration Ratio (SAAR). We aimed to create a metric for measuring antimicrobial spectrum to complement SAAR, benchmarked against local and individual resistance. METHODS: This retrospective cohort study utilised data from nine acute care hospitals within BJC Healthcare from January 2018 to December 2023. Adult patients meeting CDC sepsis event criteria were included, focussing on infections caused by Gram-negative bacilli (GNB). Data on patient characteristics, microbiology results, treatments, and timing were collected. We defined the antibiotic effective spectrum (AES) as the ratio of susceptible GNB to all GNB cultured. Mathematical optimisation identified the optimal antibiotic as the antibiotic with the narrowest effective spectrum active against the isolated GNB. We calculated the empirical (before culture results are available) and directed Optimal Spectrum Antibiotic Ratio (OSAR) as the ratio between the AES chosen clinically to the optimal AES, reported it for each hospital yearly, and compared it to SAAR and the Antibiotic Spectrum Index (ASI). Sensitivity analyses focused on empirical undertreatment (OSAR