Comparative evaluation of the MRS test. A 4 to 6 hour screening test for detecting oxacillin-resistant staphylococci

A total of 131 strains of S. aureus and 25 strains of unspeciated coagulase-negative staphylococci (CNS) initially tested by automated methods for susceptibility to oxacillin were concurrently retested using standardized disk diffusion, reference 2% NaCl-supplemented broth microdilution, oxacillin salt agar, and the MRS test (a commercially prepared broth screening method). Compared to the reference broth microdilution test results, the MRS test was 97% sensitive for S. aureus, 95% sensitive for CNS, and 100% specific for all staphylococci. Results were available in 4 hr for S. aureus and less than 6 hr for CNS. The oxacillin salt agar screen had sensitivities of 93 to 99% with a specificity of 100%. Although the disk diffusion method was the most sensitive method (100%), it was the least specific (83% for S. aureus and 80% for CNS). Differences in manufacturers' agar affected results with most discrepancies resulting in a false-resistant interpretation. Although inoculum standardization was important for accurate susceptibility test results, overinoculation alone could not account for the 30 isolates falsely-resistant to oxacillin by the Vitek AMS or Abbott MS-2. Contaminants or card-fill problems may have also have been responsible for some of the discrepancies. The MRS test was considered to be an acceptable alternative screen or a supplement to other methods for same-day testing for ORS.