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Journal of Clinical Microbiology, March 2006, p. 1101-1104, Vol. 44, No. 3
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.3.1101-1104.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Accuracy of Three Automated Systems (MicroScan WalkAway, VITEK, and VITEK 2) for Susceptibility Testing of Pseudomonas aeruginosa against Five Broad-Spectrum Beta-Lactam Agents

JMI Laboratories, North Liberty, Iowa 52317,¹ Division of Infectious Diseases, Federal University of Sao Paulo, Sao Paulo, Brazil,² Tufts University School of Medicine, Boston, Massachusetts 02111³

Received 17 October 2005/ Returned for modification 8 December 2005/ Accepted 15 December 2005

One hundred recent clinical Pseudomonas aeruginosa isolates were used to assess the quantitative (MIC) and qualitative (susceptibility category) accuracies of the MicroScan WalkAway, VITEK, and VITEK 2 automated susceptibility test systems when five-broad spectrum ß-lactams, aztreonam, cefepime, ceftazidime, imipenem, and piperacillin-tazobactam, were tested. Isolates were selected so that the MICs for the isolates overrepresented the MICs near the breakpoints to assess precisely the agreement between the results obtained with the automated systems and the results obtained by the reference tests. The categorical and MIC results from the automated systems were compared to the consensus result of three reference methods: broth microdilution, agar dilution, and disk diffusion. The consensus categorical testing (susceptibility and resistance) rates were 47 and 27%, respectively, for aztreonam; 59 and 14%, respectively, for cefepime; 44 and 43%, respectively, for ceftazidime; 71 and 19%, respectively, for imipenem; and 50 and 50%, respectively, for piperacillin-tazobactam. All systems tested exhibited a high, unacceptable level of very major (false-susceptible) errors for piperacillin-tazobactam (19 to 27%). Major (false-resistant) error rates were generally acceptable (0 to 3%), but minor error rates were elevated (8 to 32%) for cefepime (VITEK 2 and VITEK) and for aztreonam (all three systems), leading to consistent trends toward false resistance. Manufacturer reevaluation of these automated systems for the testing of selected ß-lactams with current clinical isolates of P. aeruginosa that exhibit contemporary resistance mechanisms would be prudent to minimize the potential for serious reporting errors.

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