Comparison of Four Carbapenemase Detection Methods for Variants.

Recently, various variants resistant to ceftazidime-avibactam have begun to emerge in clinical settings, but it is unclear which testing method is most appropriate for detecting these variants. Strains were subjected to antimicrobial susceptibility testing using the broth microdilution method. Four carbapenemase detection methods, modified carbapenem inactivation method (mCIM) and EDTA carbapenem inactivation method (eCIM), APB/EDTA (carbapenemase inhibitor APB [3-aminophenylboronic acid] and EDTA enhancement method), NG-test Carba 5, and GeneXpert Carba-R were used to try to detect KPC-2 variants in 19 Klebsiella pneumoniae isolates. Among those variants, -, -, -, -, -, and -positive isolates accounted for 26.3% (5/19), 15.8% (3/19), 5.3% (1/19), % 42.1% (8/19), 5.3% (1/19), and 5.3% (1/19), respectively. All 19 K. pneumoniae carrying variants showed resistance to ceftazidime-avibactam (MICs:16 to >64 mg/L), and 14 strains were susceptible to imipenem (MICs: 0.25 to 1 mg/L). None of the variants could be detected using either the mCIM or the APB/EDTA method, while five strains carrying variants (, , and ) tested KPC positive when using NG-test Carba 5. However, GeneXpert Carba-R was able to detect variants (harboring , , , , , and ) carried by all 19 K. pneumoniae. The emergence of new KPC variants poses an increased challenge for carbapenemase detection methods, and laboratories should use the appropriate assays to accurately detect these variants. Carbapenemase detection is essential for the appropriate treatment of CRE infections. Several clinical laboratories have begun using relevant carbapenemase assays such as mCIM and eCIM, the APB/EDTA method, NG-test Carba 5, and GeneXpert Carba-R to detect carbapenemases. Nevertheless, some of these methods may have limitations for detecting variants. Additionally, there has been little relevant research on evaluate the differences between these standard methods for detecting variants. Therefore, we investigated the reliability of these classic methods for assessing 19 K. pneumoniae with variants. Our results showed that none of the variants could be detected using either the mCIM or APB/EDTA method, while five strains (harboring , ,and ) tested KPC positive when using NG-test Carba 5. GeneXpert Carba-R could detect six variants carried by all 19 K. pneumoniae. This study may be valuable for clinical laboratories in their efforts to test for various variants.