Methods for improved detection of oxacillin resistance in coagulase-negative staphylococci: Results of a multicenter study

A multilaboratory study was undertaken to determine the accuracy of the cur rent National Committee for Clinical Laboratory Standards (NCCLS) oxacillin breakpoints for broth microdilution and disk diffusion testing of coagulas e-negative staphylococci (CoNS) by using a PCR assay for mecA as the refere nce method. Fifty well-characterized strains of CoNS were tested for oxacil lin susceptibility by the NCCLS broth microdilution and disk diffusion proc edures in 11 laboratories. In addition, organisms were inoculated onto a pa ir of commercially prepared oxacillin agar screen plates containing 6 mu g of oxacillin per mi and 4% NaCl. The results of this study and of several o ther published reports suggest that, in order to reliably detect the presen ce of resistance mediated by mecA, the oxacillin MIC breakpoint for definin g resistance in CoNS should be lowered from greater than or equal to 4 to g reater than or equal to 0.5 mu g/ml and the breakpoint for susceptibility s hould be lowered from less than or equal to 2 to less than or equal to 0.25 mu g/ml. In addition, a single disk diffusion breakpoint of less than or e qual to 17 mm for resistance and greater than or equal to 18 mm for suscept ibility is suggested. Due to the poor sensitivity of the oxacillin agar scr een plate for predicting resistance in this study, this test can no longer be recommended for use with CoNS. The proposed interpretive criteria for te sting CoNS have been adopted by the NCCLS.