Title:Evaluation of the MAST indirect carbapenemase test and comparison with a modified carbapenem inactivation method for the detection of carbapenemase enzymes in Gram-negative bacteria

Author:J Creighton, C Tibbs.

Abstract:Introduction: Carbapenemase-producing Enterobacteriaceae (CPE) are no longer rarely encountered in New Zealand and worrying aspects of increasing prevalence include two hospital-associated outbreaks, infections in patients with no travel history, and a report of household transmission. It is critically important for both patient management and infection control purposes that carbapenemase-producing organisms (CPO) are rapidly and reliably detected and identified in clinical laboratories. However, this can be problematic due to the diversity of carbapenemase enzymes, the different genera they can reside in, and the difficulties of discriminating CPO from carbapenem-resistant-non-carbapenemase producers. Thus, the aim of this study was to evaluate and compare the recently released MAST indirect carbapenemase test (ICT) and a modified carbapenem inactivation method (mCIM) test, in order to determine their ability to detect carbapenemase production, and to reliably ‘rule out’ a noncarbapenemase producer.
Methods: A total of 100 non-duplicate isolates, consisting of 80 Enterobacteriaceae, 12 Pseudomonas aeruginosa, and 8 Acinetobacter baumannii were included in the study. The panel included 63 carbapenemase-producing strains and 37 non-carbapenemase producing multi-drug resistant strains. Each isolate was tested by the MAST ICT and a mCIM assay, with sensitivities and specificities determined.
Results: Both the MAST ICT and mCIM tests performed with 100% sensitivity, detecting all carbapenemase-producing strains. For the non-carbapenemase-producing strains, 3 false positive results were observed with the mCIM assay, giving a specificity of 91.9% and PPV of 95.5%. The MAST ICT was more subjective to interpret, with the assay initially producing 11 equivocal or false positive results (specificity 70.3%). Upon repeat testing, 4 strains were negative; giving a final specificity of 81.1% and PPV of 90.0%.
Conclusion: Our evaluation of the MAST ICT and a modified CIM assay found high sensitivity and specificity for both assays across a range of Gram negative bacteria. To reliably distinguish CPO from carbapenem-resistant-non- CPO, we would recommend that the mCIM is used in tandem with the MAST ICT, or with another high performing assay as such as Carba NP, rather than as stand-alone tests. Advantages of these tests include ease of use, simple to interpret, inexpensive and an ability to detect carbapenemase production, regardless of class type, in Enterobacteriaceae as well as Pseudomonas and Acinetobacter. An all-in-one test format rather than having to use multiple inhibitor-based tests, is favourable for laboratories with limited resources and experience.
Key words: indirect carbapenemase test, carbapenem inactivation method, carbapenemase, Gra
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