• Acta clinica Croatica · Mar 2019

    FALSE POSITIVE PHENOTYPIC DETECTION OF METALLO-BETA-LACTAMASES IN ACINETOBACTER BAUMANNII.

    • Branka Bedenić, Ranko Ladavac, Mirna Vranić-Ladavac, Nada Barišić, Natalie Karčić, Katherina Bernadette Sreter, Slobodan Mihaljević, Luka Bielen, Haris Car, and Nataša Beader.
    • 1Department of Microbiology, School of Medicine, University of Zagreb, Zagreb, Croatia; 2Department of Clinical and Molecular Microbiology, Zagreb University Hospital Centre, Zagreb, Croatia; 3Department of Nephrology, Pula General Hospital, Pula, Croatia; 4Department of Microbiology, Public Health Institute of Istria County, Pula, Croatia; 5Department of Clinical Immunology, Pulmonology and Rheumatology, Sestre milosrdnice University Hospital Centre, Zagreb, Croatia; 6Department of Anesthesiology, Zagreb University Hospital Centre, Zagreb, Croatia; 7Department of Internal Medicine, Zagreb University Hospital Centre, Zagreb, Croatia; 8Zagreb Secondary Medical School, Zagreb, Croatia.
    • Acta Clin Croat. 2019 Mar 1; 58 (1): 113-118.

    AbstractPhenotypic detection of metallo-β-lactamases (MBLs) in Acinetobacter (A.) baumannii is a serious challenge to clinical microbiologists. MBLs are inhibited by metal chelators such as ethylenediaminetetraacetic acid) (EDTA). Production of MBLs cannot be recognized based on resistance phenotype. Therefore, phenotypic tests using EDTA are recommended. The aim of this study was to investigate the sensitivity and specificity of inhibitor based tests (EDTA) for detection of MBL. A total of 172 A. baumannii strains (123 carbapenemase positive and 49 carbapenemase negative) were analyzed. Phenotypic detection of MBLs was performed by the combined disk test with EDTA (CDT-EDTA) and EPI-dilution test (EPI-DT). Both tests were positive in all 11 isolates possessing VIM-1 MBL, showing 100% sensitivity. However, false positive results were observed in strains with class D carbapenemases using both tests, i.e. all OXA-23 and OXA-24/40 producing organisms and most OXA-58 positive strains (77% with CDT-EDTA vs. 65% with EPI-DT). False positive results can occur because oxacillinases are converted to a less active state in the presence of EDTA, leading to augmentation of the inhibition zone around the carbapenem disk or reduction of carbapenem minimum inhibitory concentrations. This study showed high sensitivity but low specificity of phenotypic methods in the detection of MBLs.

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