Is it necessary to perform the oxacillin agar screen test for methicillin-resistant Staphylococcus aureus (MRSA) when using Vitek, MicroScan, or other commercial test systems?

Answer:

Although it is generally not necessary, there may be occasions when performing the test could be helpful. Years ago, when MRSA was first encountered in clinical laboratories, there was concern about the ability of the "new" commercial systems (particularly the rapid systems) to detect MRSA. Many opinion leaders advocated the use of oxacillin agar screen testing in parallel with automated tests until the laboratory was comfortable with the accuracy of their testing system. However, as the accuracy of these products improved, the need for such parallel testing decreased. Most commercial products perform reliably if test materials are handled and stored properly and the manufacturer's testing instructions are followed precisely. However, if there are concerns about the accuracy of a commercial test, particularly if a new system is being introduced into the laboratory, the oxacillin agar screen plate may be a reasonable test to run in parallel with the new method for a defined period of time. As shown in an article by Swenson et al. reviewed in the December 2001 "Hot Papers," neither the oxacillin agar screen test, nor most other commercial methods, are perfect.