A study was designed to assess the impact of the VITEK 2 automated system and the Advanced Expert System (AES) on the clinical laboratory of a typical university-based hospital. A total of 259 consecutive, nonduplicate isolates of Enterobacteriaceae members, Pseudomonas aeruginosa, and Staphylococcus aureus were collected and tested by the VITEK 2 system for identification and antimicrobial susceptibility testing, and the results were analyzed by the AES. The results were also analyzed by a human expert and compared to the AES analyses. Among the 259 isolates included in this study, 245 (94.6%) were definitively identified by VITEK 2, requiring little input from laboratory staff. For 194 (74.9%) isolates, no inconsistencies between the identification of the strain and the antimicrobial susceptibility determined by VITEK 2 were detected by the AES. Thus, no input from laboratory staff was required for these strains. The AES suggested one or more corrections to results obtained with 65 strains to remove inconsistencies. The human expert thought that most of these corrections were appropriate and that some resulted from a failure of the VITEK 2 system to detect certain forms of resistance. Antimicrobial phenotypes assigned to the strains by the AES for -lactams, aminoglycosides, quinolones, macrolides, tetracyclines, and glycopeptides were similar to those assigned by the human expert for 95.7 to 100% of strains. These results indicate that the VITEK 2 system and AES can provide accurate information in tests for most of the clinical isolates examined and remove the need for human analysis of results for many. Certain problems were identified in the study that should be remediable with further work on the software supporting the AES.The VITEK 2 system is a new system that automatically performs rapid identification and antimicrobial susceptibility testing on a manually prepared inoculum (1). The Advanced Expert System (AES) is designed to analyze results generated by the VITEK 2 system for biologic validity and then provide comments on the results. One important function of the AES is to look for inconsistencies between the identification of the organism and the antimicrobial susceptibility of the isolate. Another important function is to ascertain the antimicrobial phenotype of the isolate based on results of susceptibility tests. A third function is to deduce the susceptibility of the organism to drugs not tested based on its susceptibility to the antibiotics actually tested.In a previous study, the ability of the AES to correctly ascertain the -lactam phenotype of isolates of Enterobacteriaceae and Pseudomonas aeruginosa was determined using a panel of 196 strains collected worldwide which had been characterized by biochemical and molecular techniques for their -lactamase content (6). The results of that study showed that the AES correctly identified the -lactam phenotype of 183 (93.4%) of these isolates despite the inclusion of many rare phenotypes in the isolate panel. The study, however, did not assess the...
A three-phase analysis of the Vitek ESBL test and a double-disk (2-disk) test was performed to assess their ability to detect extended-spectrum -lactamases (ESBLs) in members of the family Enterobacteriaceae. In the first two phases involving detection of ESBLs in 157 strains possessing well-characterized -lactamases, sensitivity and specificity were found to be 99.5 and 100%, respectively, for the Vitek ESBL test and 98.1 and 99.4%, respectively, for the 2-disk test. In the third phase, in which the ability of each test to detect ESBLs in 295 clinical isolates was assessed, there was only one false positive (Vitek ESBL test). Across all three phases, the Vitek ESBL test was found to be much easier to perform than the 2-disk test. The latter also involved subjective interpretation of results. There were a total of 176 Escherichia coli and 157 Klebsiella pneumoniae isolates and less than 40 isolates of each of 14 other species evaluated. In a supplemental study of Klebsiella oxytoca, an organism possessing a chromosomal -lactamase similar to an ESBL, the Vitek ESBL test was found to be capable of detecting hyperproduction of this enzyme in strains of this species as well. These data indicate that the Vitek ESBL test is reliable for the detection of ESBLs in E. coli and K. pneumoniae, the two species in which ESBLs are most common, and of hyperproduction of the K. oxytoca -lactamase, a situation which engenders a level of resistance to this species similar to that seen with ESBLs.
The Advanced Expert System (AES) was used in conjunction with the VITEK 2 automated antimicrobial susceptibility test system to ascertain the β-lactam phenotypes of 196 isolates of the familyEnterobacteriaceae and the species Pseudomonas aeruginosa. These isolates represented a panel of strains that had been collected from laboratories worldwide and whose β-lactam phenotypes had been characterized by biochemical and molecular techniques. The antimicrobial susceptibility of each isolate was determined with the VITEK 2 instrument, and the results were analyzed with the AES to ascertain the β-lactam phenotype. The results were then compared to the β-lactam resistance mechanism determined by biochemical and molecular techniques. Overall, the AES was able to ascertain a β-lactam phenotype for 183 of the 196 (93.4%) isolates tested. For 111 of these 183 (60.7%) isolates, the correct β-lactam phenotype was identified definitively in a single choice by the AES, while for an additional 46 isolates (25.1%), the AES identified the correct β-lactam phenotype provisionally within two or more choices. For the remaining 26 isolates (14.2%), the β-lactam phenotype identified by the AES was incorrect. However, for a number of these isolates, the error was due to remediable problems. These results suggest that the AES is capable of accurate identification of the β-lactam phenotypes of gram-negative isolates and that certain modifications can improve its performance even further.
Population analysis was performed for 42 Escherichia coli isolates to determine whether heterogeneity of resistance was a factor in piperacillin-tazobactam category differences between agar dilution and broth microdilution. Of 20 isolates discordant between methods, 80% were heterogeneous. Of 22 isolates in agreement, 59% were homogeneous. Heterogeneity and homogeneity rates for those in agreement were significantly different from those that were discordant (P value, 0.010). Heterogeneity of resistance expression appears to be an important factor in category differences observed between broth microdilution and agar dilution for piperacillin-tazobactam. Interpretation of antimicrobial susceptibility tests can be subjective, even for established reference methods. Various in vitro methods to assess susceptibility have been developed, and conditions can vary widely during measurements. This diversity can cause differences in the metabolic status of bacterial cells and thus their reactivity to antibiotics, implying that results from different methods can vary per strain, as was recently seen in a multicenter study of Pseudomonas aeruginosa (1). Methodological differences can lead to very major errors (VMEs) (reference method resistant and comparator susceptible), which may lead to treatment failure. Major errors (MEs) (reference method susceptible and comparator resistant) can also occur and, although less serious therapeutically, can lead to disqualification of otherwise useful antimicrobials. The frequency of VMEs and MEs depends on the methods used and organism/antimicrobial combinations. However, certain drugs (especially combinations) are often problematic. Root causes for such differences are usually unknown, but clear differences between piperacillin-tazobactam (TZP) broth microdilution (BMD) and agar dilution (AD) results were observed for isolates of several species (2-5). Although differences between BMD and AD have been observed for other species, the population analysis discussed here was limited to Escherichia coli. We recently demonstrated that this aberrant phenotype can be clonal in E. coli (5). In this study, isolates were tested by population analysis (PA) in order to determine whether heterogeneity of resistance expression was a factor in the category differences observed between BMD and AD for TZP.A subset of 42 E. coli isolates was selected from a larger set of Enterobacteriaceae obtained for the purpose of studying TZP susceptibility (5). The isolates were obtained globally from as many geographically diverse locations as possible. Isolates in agreement as well as those in disagreement between BMD and AD were included. Strain characteristics are included in Table 1. BMD and AD TZP susceptibility testing was performed according to CLSI guidelines (6) and was done in triplicate. MICs were determined by examining the growth for each of the three replicates for each isolate and method. The lowest concentration that completely inhibited visible growth as detected by the unaided eye was recorded as t...
Preventing the dissemination of antimicrobial resistance depends on appropriate antibiotic stewardship and accurate antimicrobial susceptibility testing (AST). We report the international dissemination of Escherichia coli strains, showing discrepancies between reference methods when phenotypically tested for susceptibility to piperacillin/tazobactam (TZP). We demonstrate that these related strains are predisposed to problematic TZP AST interpretations.
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