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| LETTER TO THE EDITOR |
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LETTER |
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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.
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David H. Pincus
R&D Microbiology bioMérieux, Inc. 595 Anglum Road Hazelwood, Missouri 63042 Phone: (314) 731-7456 Fax: (314) 731-7454 E-mail: dave.pincus{at}na.biomerieux.com
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LETTER |
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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.
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R. Zbinden* A. Zbinden P. P. Bosshard E. C. Böttger Institut für Medizinische Mikrobiologie Universität Zürich Gloriastrasse 32 8006 Zürich, Switzerland
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| * Phone: 41 44 634 27 00 Fax: 41 44 634 49 06 E-mail: rzbinden{at}immv.unizh.ch |
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| Antimicrob. Agents Chemother. | Clin. Microbiol. Rev. |
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| Clin. Vaccine Immunol. | ALL ASM JOURNALS |
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