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

ABSTRACT

In this study, we determined the accuracy of phenotypic tests (cefoxitin agar dilution, 30µg-cefoxitin and 1µg-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 percent) harbored the mecA gene. Using 30µg-cefoxitin and 1µg-oxacillin disks we obtained 100 and 98.3 percent accuracy, respectively. In addition, when cefoxitin was used as marker in an agar dilution method, the higher accuracy (99.4 percent) was established with 8mg 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.