Accuracy of Detecting Resistance to Carbapenems among Gram Negative Rods: Comparison of Three Methods

ObjectiveTo compare the results of imipenem and meropenem susceptibility testing among multi-drug resistant (MDR) isolates of Acinetobacter spp., Pseaudomonas aeruginosa (P.aeruginosa) and members of the Enterobacteriacae.MethodsThree methods used for susceptibility testing of 210 isolates: disk diffusion (a reference method), MicroScan (MicroScan Walk Away 96 System, Dade Behring Inc. West Sacramento CA 95691, USA) and Etest (AB Biodisk Solna, Sweden).ResultsOf the 210 isolates, Acinetobacter spp. accounted for the majority of isolates [110(52.4%)] followed by P.aeruginosa, 79 (37.6%). These isolates were more prevalent from respiratory specimens 98 (46.7%), Acinetobacter spp. 60(28.6%) and P.aeruginosa 34(16.2%). The study has demonstrated discrepant results for carbapenems tested by MicroScan and Etest. For imipenem, the MicroScan exhibited 2.8 % very major error, major error was 10.1% but 3.9% by Etest for Acinetobacter spp., Other discrepant results (minor errors) were 28.7% and 33% for MicroScan and Etest, respectively. For meropenem, minor errors were higher by MicroScan (13.6%) and Etest (21%). For P.aeruginosa, very major error (1.6%) was exhibited by imipenem Etest but major errors were 23% and 30.5% for both drugs by MicroScan, respectively. Minor errors were higher for both drugs by both methods (MicroScan: 15.3% to 20.8% and Etest : 34.9% to 34.2%).ConclusionMicrobiology laboratories should consider the use of an additional confirmatory test for carbapenem susceptibility testing of clinical isolates of Acinetobacter spp. and P.aeruginosa and members of the Enterobacteriacae.