Evaluation of three variants of the carbapenem inactivation method for carbapenemase detection in Enterobacterales and description of the cCIM, supplemented with cloxacillin

Objectives To determine which of the three following phenotypic tests is the most suitable for the detection of carbapenemases in Enterobacterales: the modified carbapenem inactivation method (mCIM), zCIM (carbapenem inactivation method supplemented with zinc) or cCIM (carbapenem inactivation method with cloxacillin). Methods Each test was evaluated with a set of 199 isolates representing the epidemiological situation in German hospitals. Furthermore, we wanted to establish first data for the cCIM, and evaluate whether the addition of cloxacillin improves the discrimination between carbapenemase-producing and AmpC-overproducing isolates, which have been shown to be problematic for other CIM-based assays. Results The overall sensitivity/specificity was 96.1%/85.4% for the mCIM, 100%/94.8% for the zCIM and 94.2%/99.0% for the cCIM. For AmpC producers without carbapenemases the specificity of the cCIM was 98.6%, for the mCIM 89.0% and for the zCIM 95.9%. Conclusions The zCIM provided the best results. The cCIM improved the performance for AmpC producers without carbapenemases.