Performance of modified carbapenem inactivation method and inhibitor-based combined disk test in the detection and distinguishing of carbapenemase producing Enterobacteriaceae

Background: This study aimed to evaluate the performance of modified carbapenem inactivation method (mCIM) combined EDTA-carbapenem inactivation method (eCIM), and inhibitor-based combined disk test (CDT) in the detection and distinguishing of carbapenemase production in Enterobacteriaceae. Methods: A total of 101 nonrepetitive carbapenem insensitive Enterobacteriaceae [minimal inhibitory concentration (MIC) ≥2 µg/mL] were tested by mCIM, eCIM and CDT respectively, and the major carbapenemase genes including blaKPC, blaNDM, blaIMP, blaVIM and blaOXA-48-like genes were detected by polymerase chain reaction (PCR) as control. Results: Seventy-nine (78.2%) of isolates were found to harbour one or more carbapenemase genes by PCR, with blaKPC and blaNDM being the most common genes. OXA-48-like genes were undetectable. The coincidence rate of mCIM combined eCIM and CDT was 97.5% (77/79) and 96.2% (76/79) respectively, compared with gene detection. Both assays had a misclassification in two bla KPC+NDM -producing isolates of Klebsiella oxytoca . The sensitivity and specificity of two assays above were 100.0% vs . 95.0% and 98.4% vs . 98.4%, respectively in distinguishing serine-carbapenemase, while they were 95.1% vs . 97.6% and 100% vs . 100.0%, respectively in distinguishing metallo-carbapenemase. Conclusions: mCIM combined eCIM and the CDT are both useful tools for the reliable detection and distinguishing single serine-carbapenemase or metallo-carbapenemase, but not for mixed types.