Evaluation of reference values for phenotypic tests to detect oxacillin resistance in coagulase-negative staphylococci

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Aim: To evaluate the adequacy of the disc-diffusion test and E-test® compared with detection of mecA for coagulase-negative staphylococci isolated from blood cultures, nasal swabs and wounds. Results: Agreement between all techniques was observed in 65.7% of cases. The greatest discrepancy between mecA/susceptible E-test was observed for non-epidermidis species. A resistance breakpoint ≤19 mm using the oxacillin disc was found to best classify all coagulase-negative staphylococci isolates; Staphylococcus epidermidis, ≤19 mm (oxacillin) and ≤27 mm (cefoxitin); Staphylococcus haemolyticus and Staphylococcus capitis, ≤21 mm (oxacillin) and ≤18 mm (cefoxitin); Staphylococcus warneri, MICs ≥0.75 mg/l. Conclusion: Although no longer recommended by the Clinical Laboratory Standards Institute, we observed some cases in which only the oxacillin disc-diffusion test detected resistance. The discrepancy between phenotypic tests and mecA is probably due to heterogeneity and borderline resistance.



coagulase-negative staphylococci, oxacillin resistance, reference breakpoints

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Future Microbiology, v. 13, n. 5, p. 565-575, 2018.