Detection of Methicillin-Resistant Coagulase-Negative Staphylococci by the Vitek 2 System

Abstract

The accurate performance of the Vitek 2 GP67 card for detecting methicillin-resistant coagulase-negative staphylococci (CoNS) is not known. We prospectively determined the ability of the Vitek 2 GP67 card to accurately detect methicillin-resistant CoNS, with mecA PCR results used as the gold standard for a 4-month period in 2012. Included in the study were 240 consecutively collected nonduplicate CoNS isolates. Cefoxitin susceptibility by disk diffusion testing was determined for all isolates. We found that the three tested systems, Vitek 2 oxacillin and cefoxitin testing and cefoxitin disk susceptibility testing, lacked specificity and, in some cases, sensitivity for detecting methicillin resistance. The Vitek 2 oxacillin and cefoxitin tests had very major error rates of 4% and 8%, respectively, and major error rates of 38% and 26%, respectively. Disk cefoxitin testing gave the best performance, with very major and major error rates of 2% and 24%, respectively. The test performances were species dependent, with the greatest errors found for Staphylococcus saprophyticus. While the 2014 CLSI guidelines recommend reporting isolates that test resistant by the oxacillin MIC or cefoxitin disk test as oxacillin resistant, following such guidelines produces erroneous results, depending on the test method and bacterial species tested. Vitek 2 cefoxitin testing is not an adequate substitute for cefoxitin disk testing. For critical-source isolates, mecA PCR, rather than Vitek 2 or cefoxitin disk testing, is required for optimal antimicrobial therapy.T he detection of methicillin resistance (MR) in coagulase-negative staphylococci (CoNS) can be critically important for isolates from normally sterile sites. However, detection of MR CoNS is problematic and less reliable than the detection of MR Staphylococcus aureus (1, 2). Cefoxitin susceptibility testing has greatly improved the reliability of detecting MR S. aureus and, to a lesser extent, CoNS (3, 4). Current CLSI guidelines recommend the use of cefoxitin disk testing for detecting MR CoNS, with some evidence that cefoxitin MIC determination can serve the same purpose (3, 5). A number of studies have shown that cefoxitin disk testing of CoNS is generally sensitive but can be nonspecific (3, 4, 6, 7). The Vitek 2 (Vitek) antimicrobial susceptibility system utilizes a cefoxitin susceptibility screening assay that was previously reported to have a 98% sensitivity for MR and a 100% specificity for S. epidermidis but only 66% and 100% sensitivity and specificity, respectively, for S. hominis (6). This led our laboratory to perform cefoxitin disk testing for S. hominis isolates rather than rely on the Vitek cefoxitin test. In addition, when the Vitek oxacillin result was discordant from the Vitek cefoxitin result, cefoxitin disk testing was performed. We found, however, that there were a large number of discrepancies between the Vitek cefoxitin and disk cefoxitin tests. Over the period from January to December 2011, we encountered 25 Vitek cefoxitin-susceptible CoNS isolates t...

Improved Detection of mecA-Mediated β-Lactam Resistance in Staphylococcus lugdunensis Using a New Oxacillin Salt Agar Screen

“…lugdunensis. The findings are consistent with previous studies that have shown the accuracy of cefoxitin DD testing in staphylococci to be species-dependent (Ferreira et al, 2012;Johnson et al, 2014;Wu et al, 2016;Huse et al, 2018;Morris and Simner, 2019). In this study, the accuracy (category agreement) of the cefoxitin DD test was 96%.…”

Evaluation of Oxacillin and Cefoxitin Disk Diffusion and MIC Breakpoints Established by the Clinical and Laboratory Standards Institute for Detection of mecA -Mediated Oxacillin Resistance in Staphylococcus schleiferi

“…In a study of 170 isolates of mecA-positive CoNS, cefoxitin DD testing failed to identify five mecA-positive isolates of S. simulans (33). For S. saprophyticus, cefoxitin DD testing was 100% sensitive but only 56% specific compared to mecA PCR (40). Finally, the VME rate for cefoxitin DD testing for S. pseudintermedius was 29.7% compared to the mecA PCR results (34).…”

Evaluation of Oxacillin and Cefoxitin Disk and MIC Breakpoints for Prediction of Methicillin Resistance in Human and Veterinary Isolates of Staphylococcus intermedius Group

“…It should be noted that we incubated all tests for 24 h, which is longer than the recommended 16-to 20-h incubation endorsed by EUCAST (26)-we cannot exclude the possibility that the additional 4 h of incubation contributed to some MEs. Nonetheless, it is clear that neither the cefoxitin nor the oxacillin test will adequately detect mecA-mediated resistance in all species of Staphylococcus-both VMEs and MEs (32) have been documented. In particular, the cefoxitin disk test performs poorly with S. saprophyticus (32) and S. simulans (33) when results are inter- preted by using the CLSI CoNS breakpoints.…”

“…Nonetheless, it is clear that neither the cefoxitin nor the oxacillin test will adequately detect mecA-mediated resistance in all species of Staphylococcus-both VMEs and MEs (32) have been documented. In particular, the cefoxitin disk test performs poorly with S. saprophyticus (32) and S. simulans (33) when results are inter- preted by using the CLSI CoNS breakpoints. The mechanisms behind these discrepancies are unclear, but the mechanism of beta-lactam resistance in the staphylococci is complex, involving the participation of both acquired and native PBP-encoding genes; therefore, differences in phenotypic resistance among the various staphylococcal species are not unanticipated (34).…”