274 - Comparison of Cefazolin Susceptibilities of Enterobacterales with an Automated Susceptibility Testing Platform versus In Vitro Antimicrobial Testing

Background: The Clinical and Laboratory Standards Institute (CLSI) revised breakpoints for cefazolin (CFZ) may be difficult to implement with current automated susceptibility testing (AST) platforms and Enterobacterales may be falsely reported as susceptible to CFZ. The possibility remains that CFZ may then be inappropriately used as definitive therapy.

Methods: This was a retrospective observational cohort of adult patients with Enterobacterales bloodstream infections (BSI) reported CFZ susceptible per Vitek^®2 (bioMerieux, Durham NC). The primary outcome was the percentage of CFZ susceptible Enterobacterales isolates using three different susceptibility testing methods: Vitek^®2 automated testing, ETEST^® (bioMerieux, Durham NC), and disk diffusion. Secondary outcomes included treatment failure defined as a composite outcome of 30-day all-cause inpatient mortality, 30-day recurrent BSI, 60-day recurrent infection, or infectious complications.

Results: In 195 isolates reported CFZ susceptible per Vitek^®2, 84 (43.1%) were CFZ susceptible using E-test vs.119 (61%) using disk diffusion (Figure 1). Rates of treatment failure were similar in both CFZ and non-CFZ groups (33.3% vs. 38.5% respectively; p=0.57). Both groups had high rates of ID consult involvement ( >60%) and source control ( >80%) with urinary tract being the most reported source. No difference was noted in 30-day all-cause mortality, secondary infectious complications, 30-day readmissions, or 60-day recurrent infections. A subgroup analysis of patients receiving CFZ vs. ceftriaxone suggests treatment failure was significantly less likely to occur in the setting of source control (adjusted OR 0.06; 95% CI, 0.13-0.32) and ID consultFigure 1: CFZ Susceptibilities by Testing Method


Conclusion: There was a large discrepancy among testing methods; additional confirmatory CFZ susceptibility testing beyond AST platforms should be considered prior to definitive use of CFZ for systemic Enterobacterales infections.