What type of study is this?

This is a In Vitro Study study.

What question did this study set out to answer?

To evaluate microbiologic and clinical success of levofloxacin in patients with levofloxacin-susceptible, ciprofloxacin-nonsusceptible Enterobacterales.

January 14, 2026Open Access

P-1331. Efficacy of Levofloxacin for Levofloxacin-Susceptible, Ciprofloxacin-Nonsusceptible Enterobacterales

Key Points

To evaluate microbiologic and clinical success of levofloxacin in patients with levofloxacin-susceptible, ciprofloxacin-nonsusceptible Enterobacterales.
Single-arm, retrospective analysis of patients aged 18-89 treated for levofloxacin-susceptible, ciprofloxacin-nonsusceptible Enterobacterales
Susceptibility testing via Phoenix BMD and Kirby Bauer for Serratia
Primary outcome was 90-day microbiologic failure
Secondary outcomes included growth of levofloxacin-resistant isolates and 90-day all-cause mortality.
Microbiologic failure occurred in 29.2% of patients
Levofloxacin-resistant isolates grew in 4 patients
Treatment failure was observed in 22.9% of patients
90-day all-cause mortality rate was 14.6%

Abstract

Abstract Background Fluoroquinolones (FQ) are critical Gram-negative agents due to their oral availability and stability against beta-lactamases such as ESBL and AmpC. Resistant organisms frequently co-harbor FQ resistance genes. There are copious resistance mechanisms and no widely available genotypic analysis, yet in vitro studies suggest a higher resistance risk for discordant susceptibilities. Discordance is documented in a subset of Enterobacterales isolates; there is a paucity of literature describing FQ efficacy in this population. We aim to evaluate microbiologic and clinical success of patients receiving levofloxacin LVX-Susceptible (LVX-S), Ciprofloxacin-nonsusceptible (CIP-NS) Enterobacterales.Table 1.Published Microbiologic Success in Similar Populations Methods This is a single-arm, retrospective, descriptive analysis of patients aged 18-89 treated for LVX-S, CIP-NS Enterobacterales who received LVX. Susceptibility testing was performed via Phoenix BMD with the exception of Serratia (Kirby Bauer). Primary outcome was 90d microbiologic failure, defined as growth of the same organism within 90d of index culture. Secondary outcomes were growth of LVX-R isolates, 90d treatment failure defined as retreatment regardless of cultures, receipt of salvage therapy, and 90d all-cause mortality. Results Preliminary reporting identified 73 patients; 48 were included. Median age was 60.0 and patients were predominantly female (66.7%). Most common organism was K. pneumoniae (41.7%). The majority (85.4%) were ceftriaxone-R. Microbiologic failure occurred in 14 (29.2%) of patients. LVX-R isolates grew in 4 patients. Treatment failure occurred in 11 (22.9%) patients. 90d all-cause mortality was 14.6%. Post hoc analysis of risk factors for composite failure was not significant, including receipt of high-dose LVX (OR = 0.2; 95% CI: 0.1-1.1). Conclusion In this cohort of LVX-treated patients with LVX-S, CIP-NS isolates showed 90d microbiologic failure in 29.2% of patients, which is higher than published literature. Lower FQ dose may be a risk factor for failure. This study has significant limitations including its design and small sample size. To our knowledge, this is the first study of clinical outcomes for FQ-treated patients with discordant FQ sensitivities. Results should be confirmed with larger analyses. Disclosures All Authors: No reported disclosures

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