Comparative Evaluation of Two PCR-Based Methods for Detection of Methicillin-Resistant Staphylococcus aureus (MRSA): Xpert MRSA Gen 3 and BD-Max MRSA XT

Abstract

We compared two walk-away molecular diagnostic assays, the GeneXpert MRSA Gen 3 assay and the BD-Max MRSA XT assay. A total of 119 prospective swabs and 36 culture-positive samples were tested. Xpert MRSA Gen 3 had sensitivity of 95.7% and specificity of 100% versus 87.5% and 97.1% for BD-Max. The difference in agreement with the enriched culture results was significantly in favor of the Xpert assay (P < 0.02, McNemar nonparametric text).

FIG 1

Representative MREJ fluorescence curves generated by the BD-Max MRSA XT in the same amplification and detection run. (A) “Backgrounded” PCR without true positive. (B) “Color-compensated” PCR without true positive. (C) “Backgrounded” PCR with true positive. (D) “Color-compensated” PCR with true positive. A10, MRSA true-negative sample; B3, MRSA true-positive sample; B9, MRSA false-positive sample. The software calls a positive MRSA result in the absence of amplification in the MREJ channel as determined by naked-eye interpretation of the curves. (A) The absence of fluorescence units on the graph and the autoscaling make the call difficult in the backgrounded PCR view in the absence of a positive result in the run. (C and D) In the presence of a true positive (sample B3), the lack of amplification of MREJ in the false positive (sample B9) is readily apparent.

FIG 2

Flow chart of the clinical samples included in the study according to ChromID culture status. The top half of each box includes results from the prospective arm of the study. The lower half includes results from the culture-positive samples, only for the MRSA culture-positive specimens. Overall numbers are shown in the circle left at the left side of the box. Unresolved samples (1 for Xpert, 4 for BD-Max) are excluded from the analysis.