Evaluation of the accuracy of various phenotypic tests to detect oxacillin resistance in coagulase-negative staphylococci.

In this study, we determined the accuracy of phenotypic tests (cefoxitin agar dilution, 30 microg-cefoxitin and 1 microg-oxacillin disks) to detect the oxacillin resistance/mecA gene among coagulase-negative staphylococci (CoNS) isolates. The presence of the mecA gene was detected by PCR technique (gold standard). A total of 176 CoNS isolates from blood of hospitalized patients were evaluated. Of these, 138 (78.4%) harbored the mecA gene. Using 30 microg-cefoxitin and 1 microg-oxacillin disks we obtained 100 and 98.3% accuracy, respectively. In addition, when cefoxitin was used as marker in an agar dilution method, the higher accuracy (99.4%) was established with 8 mg cefoxitin per mL breakpoint. Thus, despite of the agar dilution method using cefoxitin as a marker not being standard for this detection, our results suggested that it is an excellent alternative to detect the oxacillin resistance/mecA gene among CoNS isolates.