Three selective chromogenic agar plates (BBL CHROMagar MRSA II, MRSASelect, and Spectra MRSA) are all reliable screening methods for the increasingly vital job of detecting MRSA from nasal specimens, according to a study presented at the Interscience Conference on Antimicrobial Agents and Chemotherapy in Chicago. Gerald A. Denys, PhD, Clinical Director of Microbiology, Indiana University Health Pathology Lab, Indianapolis, Indiana (USA), and colleagues presented the results of their study which was designed to determine the performance of these three commercially available chromogenic media. While the three were all reliable, the researchers noted that the chromogenic approach yielded false positives due to color interpretation.
The use of chromogenic media has several advantages over the use of non-selective TSA II. These include shorter time to detection of MRSA, minimal technician time, as well as no additional need for antimicrobial susceptibility or screening tests for mecA mediated oxacillin resistance.
When the researchers compared the overall sensitivity and specificity of the three selective chromogenic agar plates to non-selective TSA II, they found that it was greater than 90%. They could not explain the differences in sensitivity between the assays. They did not feel that it was due to innoculum effect, since the order of inoculation was consistently rotated. Moreover, the recovery of MRSA by semi-quantitative culture validated the use of the single swabs for comparison in the study.
The CMRSA II was more specific than the Spectra MRSA and MRSASelect, the researchers found. They determined that the false positives on Spectra MRSA and MRSASelect were due to the interpretation of small or suspiciously colored colonies as positive. A secondary analysis based on colony color intensity showed increased specificity for Spectra MRSA and MRSASelect, when more intense blue or pink was used to differentiate MRSA. The researchers noted that questionable MRSA colonies should be confirmed by a coagulase test and susceptibility testing but that this process would lead to a delay in reporting.