Use of CultureSwab Plus Swabs with Amies Gel Agar with the IDI-MRSATM Assay

REVISED ABSTRACT

The IDI-MRSATM assay (GeneOhm Sciences, San Diego, CA) is an FDA-cleared test for the direct detection of nasal colonization with methicillin-resistant Staphylococcus aureus (MRSA). Many institutions, including ours, selectively or broadly screen new admissions for MRSA nasal colonization to facilitate appropriate isolation procedures for MRSA carriers. Utilized in conjunction with the Smart CyclerTM instrument (Cepheid, Sunnyvale, CA), the IDI-MRSATM assay provides rapid real-time results. The product insert specifies that specimens to be tested with the IDI-MRSATM assay be collected with a swab with liquid Stuart media such as the BBL CultureSwab. In our laboratory, we routinely utilize the BBL CultureSwab Plus, an Amies gel swab collection device. The purpose of this study was to compare the use of the CultureSwab and the CultureSwab Plus to detect MRSA using the IDI-MRSATM assay. The standard IDIMRSATM protocol used for swabs with liquid Stuart media was modified slightly for use with agar gel swabs with the addition of one extra processing step, heating at 95 +/- 2C for 2 minutes. The remainder of the processing was identical to the protocol used for swabs with liquid media. Preliminary work in our laboratory with the IDI-MRSATM assay indicated that the sensitivity of the assay was generally between 102 and 103 CFU. For this evaluation, we prepared dilutions with intended final concentrations of 100, 500, and 1000 CFU that were used to inoculate CultureSwab swabs with liquid Stuart’s media and CultureSwab Plus swabs with Amies gel agar media. A total of 6 swabs were inoculated for each isolate (2 media at 3 dilutions).120 recent clinical isolates were tested with 274 out of a possible 360 positive test results with the Culture Swab Plus swabs with Amies gel and 303 out of a possible 360 positive test results for the CultureSwab swabs with liquid Stuart’s media (p=0.0003). These results suggest that the CultureSwab Plus swabs with Amies gel are an acceptable alternative to the CultureSwab swabs with liquid Stuart’s media for use in the IDI-MRSATM assay but may not be as sensitive when specimens contain lower concentrations of organisms.

MATERIALS AND METHODS

• 120 recent unique patient clinical isolates of Staphylococcus aureus were used for this study.
• Dilutions were made to prepare 3 inocula with targeted concentrations of 10(4), 5 x 10(3) and 10(3) per ml.
• 0.1 ml was added to each swab. A total of 6 swabs were inoculated for each isolate (2 media at 3 dilutions) using a microtiter plate. Final targeted inocula were 100, 500, and 1000 CFU/swab.
• Processing. Liquid Stuart gel swabs were processed following instructions in the package insert. For the agar gel swabs, an additional step was added

CONCLUSIONS

• Testing of gel swabs with the IDI-MRSATM assay requires only a minor modification of the protocol used for the liquid swabs.
• The liquid swab is more sensitive than the gel swab for detection of MRSA when lower concentration of inocula are used.
• The clinical significance of the differences in vitro sensitivity is not known. We stressed the assay/swab system by testing at inocula concentrations close to the limits of sensitivity of the IDIMRSATM assay.