Evaluation of the carbapenem inactivation method (CIM) for detecting carbapenemase activity in enterobacteria

Abstract Objectives The objective of this study was to evaluate the accuracy of the CIM test in the detection of carbapenemase activity in 124 strains of Enterobacteriaceae. Methods A panel of 124 previously characterized Enterobacteriaceae was tested: 77 strains producing the following carbapenemase families: KPC ( n = 14), GES ( n = 22), NDM ( n = 19), VIM ( n = 4), IMP ( n = 4) and OXA-48 ( n = 14) and 47 non-carbapenemase producers. For the CIM method, an active susceptibility meropenem disc was exposed to a bacterial suspension of a test strain; when a carbapenemase is produced, the antibiotic is inactivated allowing uninhibited growth of an indicator strain after overnight incubation. A clear inhibition zone (≥20 mm) was considered indicative of no-carbapenemase activity. Results All KPC, NDM, VIM, IMP or OXA-48 producing strains were unequivocally detected with the CIM test. CIM false negative results were obtained with eleven Enterobacter cloacae producing GES-6. Two other E. cloacae not producing carbapenemase (one with SHV-12, one hyperproducing AmpC) were positive by the test. The sensitivity and specificity of the assay compared to those of molecular methods were 85.7% and 95.7%, respectively. Conclusions The CIM method proved to be inexpensive and easy to interpret. It provided less than optimal results in the detection of GES-6 activity.