Variation from standards in Staphylococcus aureus susceptibility testing.

In an effort to assess the degree of methodologic variation and adherence to current guidelines for detection of methicillin-resistant Staphylococcus aureus (MRSA), the authors surveyed the susceptibility testing practices of all 162 microbiology laboratories in the Veterans Administration (VA) system. Completed questionnaires were returned by 136 (84%) of the laboratories. Overall, 96 (71%) laboratories used disk diffusion testing, 54 (40%) used manual broth dilution, and 36 (26%) used an automated method. The percentage of MRSA detected ranged from 0 to 52%, with a mean of 10%. In general, fewer than 60% of laboratories followed the current susceptibility testing guidelines for key methodologic variables such as inoculum preparation, duration of incubation, and medium supplementation. Failure to adhere to these guidelines may result in suboptimal detection of MRSA.