Is there a need to revise the antibiotic concentration in Clinical and Laboratory Standards Institute-Recommended Oxacillin Screen Agar?

Introduction: In routine diagnostic microbiology laboratories, Clinical and Laboratory Standards Institute (CLSI) recommends the use of cefoxitin disc, in addition to oxacillin screen agar (OSA) of 6 μg/ml for the detection of methicillin-resistant Staphylococcus aureus (MRSA), whereas minimum inhibitory concentration values of oxacillin for S. aureus are ≤2 μg/ml (susceptible) and ≥4 μg/ml (resistant). Hence, the study was carried out to evaluate the ability of screen agar with lower concentrations of oxacillin to identify the isolates of MRSA and to compare this with cefoxitin disc diffusion (CDD).

Materials And Methods: Six hundred and seventy-six isolates of S. aureus were screened for methicillin resistance by OSA with 2 μg/ml and 4 μg/ml and 6 μg/ml of oxacillin concentration as well as CDD. Polymerase chain reaction for mecA gene was carried out for all isolates which grew on OSA 2, 4 and 6 μg/ml regardless of their cefoxitin susceptibility. Latex agglutination test for penicillin-binding protein 2a was performed for the isolates which grew on OSA 2 and or 4 μg/ml but not on OSA 6 μg/ml.

Results: Eight per cent of MRSA isolates was missed by using OSA 6 μg/ml, when compared with other methods. Sensitivities of OSA 2 μg/ml, OSA 6 μg/ml and CDD were found to be 100%, 92.5% and 97.5%, respectively, and specificities for the same were found to be 100%, 100% and 98%, respectively. As per FDA criteria, categorical agreement for OSA 2 μg/ml was found to be 100% in comparison with the reference broth microdilution method. No major and very major discrepancies were documented.

Conclusion: Similar findings on a larger and more heterogeneous collection of isolates may indicate the need to revise the concentration of OSA to 2 μg/ml for the detection of MRSA.