Evaluation of several phenotypic methods for the detection of carbapenemase-producing Pseudomonas aeruginosa

The purpose of this investigation was to compare several phenotypic methods, including combined disk tests (CDT) containing metallo-beta-lactamase (MBL) inhibitors or cloxacillin, and the Carba NP test for the detection of carbapenemase-producing Pseudomonas aeruginosa (CPPA). A new CDT using imipenem (10 mu g) +/- cloxacillin 4,000 mu g and the Carba NP test were evaluated to detect CPPA. In addition, four commercially available combined disks containing a carbapenem and ethylene-diamine-tetra-acetic acid (EDTA) or dipicolinic acid (DPA) as the inhibitor were tested in order to detect MBL-positive P. aeruginosa. All these phenotypic methods were evaluated on 188 imipenem non-susceptible P. aeruginosa (CPPA, n = 75) isolates divided into 26 well-characterized collection strains and 162 non-duplicate clinical isolates referred to the national reference laboratory in 2013. For the total of 188 isolates tested, CDT containing EDTA or DPA displayed high sensitivities (99 %) and specificities (95 %) for detecting MBL-producing isolates. CDT with cloxacillin showed a sensitivity and specificity of 97 %/96 % compared to 88 %/99 % for the Carba NP test in order to detect CPPA. For the 162 clinical isolates, CDT containing EDTA or DPA displayed a high negative predictive value (NPV) (99 %) for detecting MBL-producing isolates. CDT with cloxacillin showed an NPV of 98 %, compared to 95 % for the Carba NP test in order to detect CPPA. In our setting, CDT associating imipenem +/- EDTA or +/- DPA performed best for the detection of MBL-producing P. aeruginosa. Imipenem/imipenem-cloxacillin test yielded good NPV to exclude the presence of MBL in imipenem non-susceptible isolates.