Comparative evaluation of eravacycline susceptibility testing methods in 587 clinical carbapenem-resistant Acinetobacter baumannii isolates: broth microdilution, MIC test strip and disc diffusion.

Objectives: The study aimed to evaluate the accuracy of different methods for determining carbapenem-resistant Acinetobacter baumannii (CRAB) susceptibility to eravacycline.

Methods: We collected 587 CRAB strains from Huashan Hospital affiliated to Fudan University between 2019 and 2023. The broth microdilution (BMD) method served as the reference standard. The susceptibility results were evaluated using the clinical breakpoints established by the Chinese Committee on Antimicrobial Susceptibility Testing (ChinaCAST) (susceptible MIC,  ≤ 1 mg/L; inhibition zone diameter,  ≥ 15 mm). The study compared the reliability of the MIC test strip (MTS) and disc diffusion (DD) methods in detecting CRAB susceptibility to eravacycline.

Results: The MICs required to inhibit 50% and 90% of CRAB growth were as follows: BMD, 0.5/1 mg/L; MTS, 0.38/0.75 mg/L. According to the ChinaCAST breakpoints, the BMD method demonstrated a 98.13% (576/587) susceptibility rate, whereas the MTS and DD methods showed susceptibility rates of 97.96% (575/587) and 97.61% (573/587), respectively. The essential agreement rate between the MTS and BMD methods was 94.55%. Categorical agreement (CA) rates for the MTS and DD methods were 99.83% and 99.49%, respectively. The major error (ME) rate for MTS was 0.17%, with no very major errors (VMEs) observed. For the DD method, the ME rate was 0.51%, also with no VMEs.

Conclusion: The MTS and DD methods demonstrated strong consistency with the BMD reference method, with CA, ME and VME rates meeting methodological evaluation criteria. Both MTS and DD methods are reliable alternatives for assessing the antibacterial activity of eravacycline in clinical microbiology laboratories.