Traditional culture methods for detecting and identifying methicillin-resistant Staphylococcus aureus (MRSA) employing mannitol salt and/or blood agar for cultivation can take up to 72 hours for isolation and identification, depending on the identification procedures utilized. Concurrent with the development of molecular assays, improvements in culture methods have also been achieved. Chromogenic agar, specific for MRSA, is employed by many laboratories. These media are both selective and differential, containing chromogenic substrates. MRSA strains utilize the substrates to produce colonies of a specific and characteristic color, minimizing the need for additional identification procedures.
Initially, these agars required 48 hours of incubation; newer formulations require only 24 hours.
Given the reduced incubation and identification requirements, what are the pros and cons of the molecular assays? The cost per test will be higher with the molecular assays than with the culture methods. Will molecular methods provide for a more efficient workflow and significant improvement in the availability of results? To some extent, this will depend on how they can be implemented within each different laboratory setting.
Both previously described molecular assays require manual specimen preparation and extraction before the sample is placed into the instrument. This hands-on work may be greater than the effort in swabbing and streaking a culture plate. How much of an obstacle this is for implementation will depend on the testing volume and available staff. In a high-volume setting, this will be a more significant factor.
Questions to consider:
- Will tests be performed as specimens come in (random access), or will specimens be accumulated and batched (batch testing)? Batching is desirable to reduce this cost if controls are required with each run.
- If testing will occur in batches, how many batches can be performed in one day? This will be heavily influenced by the instrument's capacity (throughput). For example, a single analyzer may run up to 16 samples. Are larger units available? Multiple units would be needed in a high-volume lab.
- Can they be set up on more than one shift? The greater the number and frequency of batches that can be run, the more significant improvement in turnaround time can be realized.
Given these variables, implementing a molecular assay for MRSA is not a given in each laboratory.
