BD Phoenix and Vitek 2 detection of mecA-mediated resistance in Staphylococcus aureus with cefoxitin

Abstract

The BD Phoenix (BD Diagnostics, Sparks, MD) and Vitek 2 (bioMérieux, Durham, NC) automated susceptibility testing systems have implemented the use of cefoxitin to enhance the detection of methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). To assess the impact of this change, 620 clinically significant S. aureus isolates were tested in parallel on Phoenix PMIC/ID-102 panels and Vitek 2 AST-GP66 cards. The results for oxacillin and cefoxitin generated by the automated systems were compared to those generated by two reference methods: mecA gene detection and MICs of oxacillin previously determined by broth microdilution according to CLSI guidelines. Testing of isolates with discordant results was repeated to attain a majority or consensus final result. There was 100% final agreement between the results of the two reference methods. For the 448 MRSA and 172 methicillin-susceptible S. aureus isolates tested, the rates of categorical agreement of the results obtained with the automated systems with those obtained by the reference methods were 99.8% for the Phoenix panels and 99.7% for the Vitek 2 cards. A single very major error occurred on each instrument (0.2%) with different MRSA isolates. The only major error was attributed to the Vitek 2 system overcalling oxacillin resistance. In 16 instances (9 on the Phoenix system, 7 on the Vitek 2 system), an oxacillin MIC in the susceptible range was correctly changed to resistant by the expert system on the basis of the cefoxitin result. The inclusion of cefoxitin in the Phoenix and Vitek 2 panels has optimized the detection of MRSA by both systems.