Mobile carbapenemases in Gram-negative bacilli: Modification of a simplified phenotypic method for routine laboratory...

Carbapenem resistance in Gram-negative bacilli (GNB) is a global health concern that requires accurate detection and effective treatment. Identification of isolates producing Klebsiella pneumoniae carbapenemase (KPC), New Delhi metallo-β-lactamase (NDM) or both enzymes remain difficult, and inconclusive results may compromise treatment. This study proposes the modified Simplified Carbapenem Inactivation Method with Inhibitors (isCIM) to detect carbapenemase-producing GNB. To evaluate the performance of the isCIM a total of 56 carbapenemase-producing isolates were collected. All results from the new method were evaluated against the gold-standard molecular method, polymerase chain reaction (PCR), which was used to confirm the carbapenemase genes bla_KPC and/or bla_NDM. Phenotypic evaluations were performed using the Modified Carbapenem Inactivation Method with and without EDTA (eCIM and mCIM), the Simplified Carbapenem Inactivation Method (sCIM) and Combined Disk assay (CD). Cohen's kappa coefficient was performed to assess agreement between methods. For KPC and co-producers, methods without inhibitors (mCIM and sCIM) showed perfect agreement with PCR. For NDM, mCIM showed strong agreement (0.852) with PCR, whereas sCIM had almost perfect agreement (0.964). Among inhibitor-based methods, isCIM achieved perfect agreement for KPC and co-producers (1.000). For NDM-isolates, while CD reached perfect agreement with PCR, isCIM (0.890) and eCIM (0.860) had strong agreement. Regarding isolates co-producing carbapenemases, isCIM and CD results had better performance than eCIM. The results suggest that isCIM is highly effective to detect and differentiate carbapenemases, especially in co-producing isolates. The modified method is a useful and promising tool for routine use in clinical microbiology laboratories because it is easy to perform.