Different antimicrobial susceptibility testing methods to detect ertapenem resistance in enterobacteriaceae: VITEK2, MicroScan, Etest, disk diffusion, and broth microdilution

• Performance for ertapenem resistance detection of various methods was investigated.
• Ertapenem susceptibility results were different depending on the methodology.
• Ertapenem MICs near intermediate breakpoint were susceptible to error.
• The detection of ertapenem resistance in Enterobacteriaceae has limitations.
• Confirmation of results by additional MIC tests is recommended for ertapenem.

We investigated different antimicrobial susceptibility testing methods to detect ertapenem resistance in Enterobacteriaceae. A total of 72 Enterobacteriaceae isolates were collected from a clinical microbiology laboratory of a tertiary university hospital, all of which were detected ertapenem resistance by the VITEK2 system. Bacterial identification and antimicrobial susceptibility were determined using the VITEK2. Ertapenem susceptibility test was performed using the MicroScan, Etest and a disk diffusion test. Ertapenem MICs were confirmed using the broth microdilution (BMD). Sensitivity, specificity, and positive and negative predictive values (PPV and NPV, respectively) of each method for the detection of ertapenem resistance were calculated. Carbapenemases and AmpC β-lactamase were screened using phenotypic methods. Among the 72 isolates, 20 isolates (27.8%) were resistant to ertapenem. Etest showed high sensitivity and specificity (85.0% and 88.5%, respectively) and excellent concordance with BMD. The disk diffusion test had the lowest sensitivity of 50.0%. The VITEK2 showed the lowest essential and categorical agreement (30.5% and 27.8%, respectively). The MicroScan showed relatively good agreement with BMD compared to the VITEK2. Most category disagreements were minor errors. There were 3 very major errors in both the MicroScan and disk diffusion test. Only 1 isolate was positive for carbapenemase screening test and all of the isolates were positive for AmpC screening test. In conclusion, the detection of ertapenem resistance in Enterobacteriaceae has limitations using routine testing such as an automated system or disk diffusion. Confirmation of results by an additional MIC test is recommended for accurate resistance results of ertapenem.