JAC Advance Access published online on January 28, 2008
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkn016
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Evaluation of phenotypic tests for the detection of metallo-β-lactamase-producing Pseudomonas aeruginosa in a low prevalence country
1 Reference Centre for Detection of Antimicrobial Resistance, Department of Microbiology and Infection Control, University Hospital of North Norway, Tromsø, Norway 2 Karolinska-Institutet-MTC, Karolinska University Hospital, Solna, Department of Clinical Microbiology, Stockholm, Sweden 3 Division of Infection Control, Norwegian Institute of Public Health, Oslo, Norway 4 Department of Microbiology and Virology, University of Tromsø, Tromsø, Norway
Received 12 November 2007; returned 3 January 2008; revised 6 December 2007; accepted 4 January 2008
* Corresponding author. Tel: +47-77627043/+47-97653716; Fax: +47-77627015; E-mail: orjan.samuelsen{at}unn.no
Objectives: To evaluate four phenotypic tests for the detection of metallo-β-lactamase (MBL) production in Pseudomonas aeruginosa in a low MBL prevalence setting.
Methods: Sixty clinical isolates of P. aeruginosa resistant to imipenem and/or meropenem and seven MBL-positive control strains were examined by: (i) MBL Etest; (ii) combined imipenem discs supplemented with EDTA (IPM-EDTA); (iii) β-lactam discs on dipicolinic acid plates (DF-DIPI); and (iv) the Cica-beta test. Spectrophotometric analysis of crude cell extracts for imipenem hydrolysis along with consensus PCRs for blaVIM and blaIMP was used as reference methods.
Results: Two clinical isolates (3%) were MBL-positive. The MBL Etest and IPM-EDTA test scored positive for all MBL-positive isolates, but showed specificities of 86% and 91%, and positive predictive values (PPVs) of only 20% and 29%, respectively. Adding resistance to ceftazidime (MIC >8 mg/L) as a criterion for MBL testing would reduce the number of isolates to be screened by 50% and increase the PPVs of the MBL Etest and IMP-EDTA test to 29% and 40%, respectively. The Cica-beta test correctly identified all MBL-negative isolates, but misidentified one MBL-positive clinical isolate as an extended-spectrum β-lactamase (ESBL)-producer and one as inconclusive (producing multiple β-lactamases). No reliable breakpoints could be defined for the DF-DIPI test due to overlapping inhibition zone diameters for MBL-positive and -negative isolates.
Conclusions: None of the phenotypic tests were optimal due to low sensitivity or specificity, resulting in low PPVs. Including ceftazidime resistance to the MBL-screening criteria would significantly improve the performance of the MBL Etest and IPM-EDTA disc test.
Key Words: metal-chelators , inhibitors , MBL Etest , combined disc , Cica-beta , dipicolinic acid
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