Comparison of the double-disk, combined disk, and Etest methods for detecting metallo-beta-lactamases in gram-negative bacilli.

Three phenotypic methods for detecting metallo-beta-lactamases (MBLs) in gram-negative bacilli were tested on 60 MBL producers and 155 MBL nonproducers. The 2-mercaptopropionic acid double-disk potentiation method using ceftazidime and cefepime with and without clavulanate was found to be most sensitive (100%). The combined disk method depends on comparing the zones given by disks containing a beta-lactam with and without ethylenediaminetetraacetic acid. Various drug combinations were tested, and the best results were obtained with imipenem for Pseudomonas species and Acinetobacter baumannii, ceftazidime-clavulanate for Klebsiella pneumoniae, and cefepime-clavulanate for Enterobacter cloacae and Citrobacter freundii. The overall sensitivity of the combined disk method was 86.7%. The Etest (AB BIODISK, Solna, Sweden) method using imipenem- ethylenediaminetetraacetic acid strips detected imipenem-resistant MBL-producing isolates only, resulting in a sensitivity of 36.7%. Our data suggest that the double-disk and combined disk tests are acceptable methods for MBL detection in both imipenem-resistant and imipenem-susceptible isolates.