REPLY
Our study was initially designed to determine why there was such a poor correlation between cefoxitin disk testing and Vitek 2 system (Vitek 2) cefoxitin screen results for the coagulase-negative staphylococci (CoNS) tested in our laboratory. We found two main reasons for this, inappropriate susceptibility testing of Staphylococcus saprophyticus because urine isolates of CoNS were not identified to the species level and inaccuracy of Vitek 2 for the identification of methicillin resistance. The data for all isolates tested, including S. saprophyticus, were presented in the main body of the results because that represented our practice at the time. It is important to note that Vitek 2 does not require entry of the CoNS species identity for results to be reported, including those for methicillin resistance; a warning is issued about the possible use of an incorrect oxacillin susceptibility breakpoint, but no warning is issued about possibly incorrect cefoxitin screen test results.
While exclusion of S. saprophyticus from the analysis of the performance of Vitek 2 improved the overall specificity of detection of methicillin resistance of CoNS from 60% to 84%, this did not improve test performance enough to meet clinical needs or to meet the FDA guidelines on test performance. Vitek 2 performance may be significantly better in other laboratories testing CoNS populations that are more like those used for the FDA submission. However, one other study reported Vitek 2 performance issues for the detection of methicillin-resistant CoNS (1), suggesting that our experience is not unique.
Dunne and colleagues (2) suggest that our gold standard for methicillin resistance should have been the cefoxitin disk test. Unfortunately the performance of cefoxitin disk testing of CoNS is neither as sensitive nor as specific as is mecA PCR testing (1, 3–6). We believe that our reference test method is accurate and that it should be the preferred method for determining the ability of phenotypic methods to detect methicillin resistance of CoNS.
Footnotes
This is a response to a letter by Dunne et al. (doi:10.1128/JCM.02771-14).
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