Association of minimum inhibitory concentration with outcome in patients with Stenotrophomonas maltophilia bacteraemia treated with empirical levofloxacin.

Stenotrophomonas maltophilia is intrinsically resistant to carbapenems and most β-lactam antibiotics. Empiric therapy for bloodstream infections often includes either trimethoprim-sulfamethoxazole or levofloxacin. However, there are currently no European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints available for interpretation of susceptibility testing for levofloxacin. We examined the relationship between levofloxacin minimum inhibitory concentration (MIC) and clinical outcomes in S. maltophilia bacteraemia. We retrospectively examined 79 consecutive patients with S. maltophilia bacteraemia from a single centre who received empiric antibiotic therapy with levofloxacin from 1 January 2012 to 30 June 2024. Adverse outcomes were defined as all-cause in-hospital mortality and were compared against the levofloxacin MIC, as tested by ETEST. Of the 79 patients, 28 (35.4%) experienced in-hospital mortality. A majority of infections were central-line related, which had comparatively better outcomes than pneumonia or intra-abdominal infections. A majority of infections were acquired nosocomially, and patients often had exposure to carbapenems prior to presentation. The MIC ranged from 0.125 mg/L to ≥32 mg/L, with MIC50 and MIC90 of 1 mg/L and 2 mg/L, respectively. Increasing frequency of adverse outcomes was observed at higher MIC cut-offs. Mortality was 100% at an MIC ≥4 mg/L. Adverse outcomes were common in S. maltophilia bacteraemia. In patients treated with levofloxacin, mortality increases sharply at a cut-off of ≥2 mg/L. Future prospective clinical studies are needed to verify these findings in other settings. S. maltophilia is an important nosocomial pathogen, conferring high morbidity and mortality in bloodstream infections. Despite its frequent treatment with levofloxacin, there are no EUCAST breakpoints to guide susceptibility interpretation. Our study demonstrates a clear association between increasing levofloxacin MIC and adverse outcomes, with a sharp mortality increase at an MIC ≥2 mg/L. These findings provide crucial clinical data to inform antibiotic selection and the determination of MIC breakpoints for levofloxacin in the treatment of this infection.