Comparison of ciprofloxacin and aminoglycoside susceptibility testing for ceftriaxone non-susceptible Enterobacterales by disk diffusion and VITEK 2 vs. broth microdilution using updated Clinical and Laboratory Standards Institute breakpoints.

BACKGROUND

Fluoroquinolones and aminoglycosides are potential treatment choices in the setting of increasingly multi-drug resistant Enterobacterales. The Clinical & Laboratory Standards Institute (CLSI) breakpoints for fluoroquinolones and aminoglycosides in the Enterobacterales were revised in 2019 and 2022, respectively. However, performance of existing widely used automated systems, such as the VITEK 2 AST-N391 card, has not been extensively tested for MDR isolates at these new breakpoints.

OBJECTIVE

To assess performance of the new breakpoints for ciprofloxacin, gentamicin, and tobramycin on the VITEK 2 system (bioMérieux, France) and disk diffusion by comparing to broth microdilution for ceftriaxone nonsusceptible Enterobacterales.

METHODS

Ninety-four ceftriaxone non-susceptible Escherichia coli and Klebsiella pneumoniae isolates were identified between January 2021-June 2023. Broth microdilution was used as the reference standard against which disk diffusion and VITEK 2 susceptibility testing were compared. For the Vitek 2, we used the AST-N391 card and interpreted the results according to the updated CLSI breakpoints.

RESULTS

Overall, 22.3% of isolates were susceptible to ciprofloxacin by BMD. Compared to BMD, disk diffusion had an overall minor error rate of 7.4% (95%CI 3.0-14.7%) with 0 major or very major errors (97.5% CI 0-3.8%). For the VITEK 2, a minor error rate of 13.8% (95% CI 7.6-22.5%), major error rate 19.0% (95%CI 7.7-40.0%) and very major error rate 0% (97.5%CI 0-3.8%) was noted. By comparison, 69.1% and 56.4% of isolates were susceptible to gentamicin and tobramycin, respectively. Disk diffusion and the VITEK 2 system both correctly categorized 100% of gentamicin susceptible and non-susceptible isolates. For tobramycin, disk diffusion had a 3.2% rate of misclassification (all minor errors) and the VITEK 2 had 2.1% rate of misclassification (all minor errors). There were no major or very major errors.

CONCLUSIONS

Our findings suggest that both disk diffusion and to a greater extent the AST-N391 card for the VITEK 2 system will overcall non-susceptibility according to current CLSI breakpoints for ciprofloxacin. By contrast, the existing AST-N391 VITEK 2 card can likely be used to correctly infer susceptibility to gentamicin and tobramycin.