Reliability of carbapenem resistance in Pseudomonas spp. detected by automated systems

Abstract number: p1453

Karatuna  O., Soyletir  G.


Ever since automated antimicrobial test systems took their places in microbiology laboratories, effort has been made to improve their accuracy. Even though antimicrobial susceptibility testing became much easier with automated systems, they are still far away from standard methods when it comes to accuracy of the test results. Micro organisms like Pseudomonas show multidrug resistance and accurate results for their last resort antibiotics such as carbapenems became of vital importance. In the literature false-resistant results have already been documented for carbapenems in Pseudomonas, which falsely limits therapy options.

Materials and Methods: 

In this study all Pseudomonas spp. (n: 133) isolated in our hospital between May and November 2005 have been evaluated for their carbapenem resistance. Identification and antimicrobial susceptibility testing were performed with Vitek2 system using Vitek2 GN and Vitek2 AST-GN09 cards (bioMérieux). No further tests were performed for isolates (n: 77), which were susceptible for carbapenems, imipenem and meropenem since our aim was to detect false resistance. For the remaining isolates (n: 36), which were, imipenem and/or meropenem resistant, both imipenem and meropenem MIC values have been established using E-test strips (AB BIODISK) even when one of the carbapenems was susceptible according to Vitek2.


Agreement and minor error rates between two methods for carbapenem susceptibility testing (n: 72) were 63.9% and 29.1%, respectively. We also observed discrepant results causing very major error rate of 4.2% in strains, which were intermediate or resistant to one of the drugs but susceptible to the other (Figure).


Our results exhibiting error rates out of acceptable ranges proved once again that laboratories, which use an automated system should consider using at least a second method to validate intermediate or resistant results for carbapenem group antibiotics when tested against Pseudomonas species. More importantly, finding an unacceptable rate of very major error even in a limited number of the tested strains strongly indicates that any result, either susceptible or resistant, obtained from an automated system should not be relied on.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
Back to top