New recommendations for disk diffusion antimicrobial susceptibility tests for methicillin-resistant (heteroresistant) staphylococci

Abstract

The agar disk diffusion susceptibility test was reevaluated for its ability to discriminate between susceptible and resistant Staphylococcus aureus (128 strains) and coagulase-negative staphylococci (19 strains) when tested with methicillin, oxacillin, and nafcillin. The results show that the current recommendations for disk potencies and interpretive zone diameters do not fit well with MIC correlates that we now recommend. Based on data from this study, we suggest that these parameters of the test be changed. For methicillin, we recommend a 10 μg disk with breakpoints of ≤11 mm (≥16 μg/ml) to indicate resistance and ≥15 mm (≤4 μg/ml) to indicate susceptibility. For oxacillin and nafcillin, we recommend 4-μg disks with breakpoints of ≤12 mm (≥8 μg/ml) to indicate resistance and ≥16 mm (≤2 μg/ml) to indicate susceptibility. MIC breakpoints were from a broth microdilution system which used a medium containing salt. If one of these three penicillins were to be selected for routine tests, we would recommend oxacillin, based on our data, but we recognize that this may depend upon the population of staphylococci within a particular hospital.