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The recent publication of Zbinden et al. (4) regarding the Vitek 2 colorimetric GN card for the identification of gram-negative nonfermentative bacilli contains data inconsistencies which the authors should have addressed before making recommendations for the use of this product.
The authors stated that the 90 isolates tested were “well identified” but that 20/90 (22.2%) were not identified to the species level. Their “gold standard” reference method (partial 16S rRNA gene sequencing) failed to characterize 10 strains beyond the genus level, and the remaining 10 strains could not be resolved between two species.
The authors recommended the Vitek 2 GN card for only five nonfermentative taxa—Achromobacter xylosoxidans, Acinetobacter spp., Burkholderia cepacia complex, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia—but the authors lacked sufficient data to contraindicate the use of the card for other species claimed (identified) by this product. While the Vitek 2 GN card claims over 60 nonfermenter species, the authors tested only 35 taxa, of which at least 14 represented unclaimed taxa; all things considered, less than one-third of the claimed species were actually tested. Of the species whose claims were questioned, over 10 were represented by testing only a single strain. These results appear to lack statistical significance.
As mentioned above, at least 14/90 (15.6%) study isolates represented species for which there is no product claim. From the data shown in Table 2 of the article (4), one cannot determine the actual number of unclaimed taxa that were tested, but it appears to be at least 15 (16.7%), if one excludes the three unclaimed Acinetobacter spp. There is no commercial identification product that can give the correct result for an unclaimed taxon. With this in mind, it is purely a subjective choice by the authors as to whether unclaimed taxa are included in their study and, if so, to what extent. Clearly, that choice can bias and affect the overall product performance.
In the article by Zbinden et al. (4), the totals shown in Table 1 are not an obvious translation of the individual data shown in Table 2. Additionally, there was no protocol in place to resolve discrepancies and verify whether the correct culture was tested. The “not identified” category members (n = 9 isolates) in Table 1 were erroneously grouped with and considered part of the “misidentified” category in the abstract. Additionally, the authors categorized low-selectivity results (n = 5 isolates) in which more than one genus was listed as “not identified” in Table 1. However, these results represent correct identifications to the species level once supplemental testing is performed.
The authors recommended the use of the GN card with five of the most commonly encountered nonfermenters in a clinical lab, but they also made recommendations regarding the qualitative level of identification that were unfounded based on their data. Based on their recommendation, one would discount the reliability of the “acceptable” identifications by the Vitek 2 fluorescent card that proved to be 100% (3/3) correct to the species level in Table 3 of the article (4).
A review of the literature shows that other investigators (1-3) found the Vitek 2 GN card to be very reliable for the identification of gram-negative nonfermentative bacilli. Correct results were obtained for 98.7%, 100.0%, and 92.1% of 144, 95, and 88 isolates studied, respectively. It is, therefore, curious how disparate the results of this study were, unless something went awry with the authors' experimental design.
REFERENCES
1.Funke, G., and P. Funke-Kissling. 2004. Evaluation of the new VITEK 2 card for identification of clinically relevant gram-negative rods. J. Clin. Microbiol. 42:4067-4071. [DOI] [PMC free article] [PubMed] [Google Scholar]
2.Renaud, F. N. R., E. Bergeron, S. Tigaud, C. Fuhrmann, B. Gravagna, and J. Freney. 2005. Evaluation of the new Vitek 2 GN card for the identification of gram-negative bacilli frequently encountered in clinical laboratories. Eur. J. Clin. Microbiol. Infect. Dis. 24:671-676. [DOI] [PubMed] [Google Scholar]
3.Wallet, F., C. Loïez, E. Renaux, N. Lemaitre, and R. J. Courcol. 2005. Performances of VITEK 2 colorimetric cards for identification of gram-positive and gram-negative bacteria. J. Clin. Microbiol. 43:4402-4406. [DOI] [PMC free article] [PubMed] [Google Scholar]
4.Zbinden, A., E. C. Böttger, P. P. Bosshard, and R. Zbinden. 2007. Evaluation of the colorimetric VITEK 2 card for identification of gram-negative nonfermentative rods: comparison to 16S rRNA gene sequencing. J. Clin. Microbiol. 45:2270-2273. [DOI] [PMC free article] [PubMed] [Google Scholar]
We appreciate the interest of Dr. Pincus in our evaluation. While we realize the need for industry-sponsored studies (2, 3) and the producer's interest in a favorable study outcome, we do note a general misperception. Our study (4) did not intend to test the accuracy of the Vitek 2 colorimetric GN card on an arbitrary strain collection. Rather, our study was designed to define the utility of the GN card in the setting of a clinical diagnostic laboratory. Consequently, we did not exclude clinical isolates that are not in the database of Vitek 2, so as not to bias our study in favor of the diagnostic procedure to be evaluated. Given the need for cost-efficient procedures in diagnostic microbiology, we deliberately excluded clinical isolates of P. aeruginosa from the study, as corresponding strains are easily identified visually. For our evaluation, we used 90 of 107 clinical isolates from a previous study (1) which was “designed to prospectively compare phenotypic with molecular identification for clinically relevant isolates of aerobic nonfermenting gram-negative (non-Pseudomonas aeruginosa) rods. The isolates (n = 107) investigated were recovered from blood cultures and from relevant clinical material where identification was required. During the study period of 26 months, a total of 2,653 aerobic gram-negative nonfermenters were isolated in our laboratory, including 1,893 strains of Pseudomonas aeruginosa; of the 760 non-P. aeruginosa isolates, 107 strains were found to meet the criterion of clinical relevance and were thus included in the study.” Thirteen of the 107 isolates could not be subcultured, and four were excluded as the final identification results remained unresolved, reducing the number of clinical strains investigated in the current study to 90 (4).
On the basis of our results, we did not suggest limiting the use of the Vitek 2 colorimetric GN card to restricted taxa. Rather, we recommended that any identified taxon other than A. xylosoxidans, Acinetobacter spp., B. cepacia complex, P. aeruginosa, and S. maltophilia should be subjected to 16S rRNA gene sequencing when accurate identification of nonfermentative rods is of concern. We explicitly stated that the numbers of representatives of other taxa classified as excellent or very good identifications by the GN card were too small to allow for sound conclusions. Our experience in the last 2 years confirms the algorithm suggested, and by accumulating more representatives from other taxa, our data also indicate that additional species, besides those pointed out above, may be identified quite satisfactorily by the colorimetric GN card, e.g., Ralstonia pickettii. It is debatable whether the four strains mentioned by Dr. Pincus, which were not identified by the colorimetric Vitek 2 GN card, should have been separated from the 28 (31%) misidentified strains in Table 1 of our article (4). Most of the five strains with low-selectivity identification results were categorized as “not identified,” as the taxa suggested were different from the final identifications—supplemental phenotypic tests would not have corrected these primarily false assignments.
There is no need to question the experimental design or the outcome of our study. We stand behind our data. We would like to encourage the diagnostic industry to respond to the need for studies evaluating the usefulness of commercial diagnostics under real-life laboratory conditions, rather than to refer to studies based on strain collections.
REFERENCES
1.Bosshard, P. P., R. Zbinden, S. Abels, B. Böddinghaus, M. Altwegg, and E. C. Böttger. 2006. 16S rRNA gene sequencing versus the API 20 NE system and the VITEK 2 ID-GNB card for identification of nonfermenting gram-negative bacteria in the clinical laboratory. J. Clin. Microbiol. 44:1359-1366. [DOI] [PMC free article] [PubMed] [Google Scholar]
2.Funke, G., and P. Funke-Kissling. 2004. Evaluation of the new VITEK 2 card for identification of clinically relevant gram-negative rods. J. Clin. Microbiol. 42:4067-4071. [DOI] [PMC free article] [PubMed] [Google Scholar]
3.Wallet, F., C. Loïez, E. Renaux, N. Lemaitre, and R. J. Courcol. 2005. Performances of VITEK 2 colorimetric cards for identification of gram-positive and gram-negative bacteria. J. Clin. Microbiol. 43:4402-4406. [DOI] [PMC free article] [PubMed] [Google Scholar]
4.Zbinden, A., E. C. Böttger, P. P. Bosshard, and R. Zbinden. 2007. Evaluation of the colorimetric VITEK 2 card for identification of gram-negative nonfermentative rods: comparison to 16S rRNA gene sequencing. J. Clin. Microbiol. 45:2270-2273. [DOI] [PMC free article] [PubMed] [Google Scholar]
