Evaluation of the Xpert methicillin-resistant Staphylococcus aureus (MRSA) assay using the GeneXpert real-time PCR platform for rapid detection of MRSA from screening specimens

Abstract

The need for rapid methods to accurately detect methicillin-resistant Staphylococcus aureus (MRSA) is widely acknowledged, and a number of molecular assays are commercially available. This study evaluated the Xpert MRSA assay, which is run on the GeneXpert real-time PCR platform (Cepheid) for use in a clinical laboratory. The following parameters were investigated: (i) the limits of detection (LoDs) for four MRSA strains; (ii) the ability to detect isolates of MRSA from a collection representative of MRSA in Ireland since 1974 (n = 114) and the ability to detect control strains with staphylococcal cassette chromosome mec types IVa (IV.1.1.1), IVb (IV.2.1.1), IVc (IV.3.1.1), IVd (IV.4.1.1), V (V.1.1.1), V(T), and VI; and (iii) performance in a clinical trial with swabs from nose, throat, and groin/perineum sites from 204 patients, where results were compared with those obtained by direct and enrichment cultures. The average LoD of the four test strains was 610 CFU/ml (equivalent to 58 CFU/swab). All 114 MRSA isolates and 7 control strains tested were detected. Sensitivity, specificity, and positive and negative predictive values for clinical specimens from all sites investigated were 90%, 97%, 86%, and 98%, respectively, but throat specimens yielded poor sensitivity (75%). Sensitivity, specificity, and positive and negative predictive values for nasal specimens were 95%, 98%, 90%, and 99%, respectively. Overall, the assay was rapid and easy to perform, but performance might be enhanced by the inclusion of an equivocal interpretive category based on analysis of all available amplification data.

FIG. 1.

Amplification data obtained with the Xpert MRSA kit for two of the four MRSA isolates used in LoD assays (A and B), two replicates of S. aureus ATCC 25923 (C and D), and two replicates of an MRSA isolate (tested at 10⁶ CFU/ml) recovered from a specimen yielding a kit-negative result (E and F). All isolates demonstrated kit-negative results with appropriate amplification of the internal control. The amplification plots in panels A and B demonstrate an exponential rise in fluorescence emission that is consistent with amplification of the MRSA target, suggesting false-negative kit results. The amplification data shown in panel C are representative of results obtained with most replicates of S. aureus ATCC 25923, but on some occasions this MSSA isolate yielded a plot suggestive of MRSA target amplification (panel D). The data shown in panels E and F were obtained from an MRSA isolate tested at a bacterial concentration of 10⁶ CFU/ml on two separate occasions. While both replicates yielded kit-negative results, one replicate demonstrated evidence of MRSA target amplification (panel F).