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.

Detection of Methicillin-Resistant Staphylococcus aureus by a Duplex Droplet Digital PCR Assay

“…The LoDs for the Cepheid GeneXpert have been reported for four strains of MRSA collected on Copan swabs and were compared to values for direct and enrichment cultures employing Transwabs with Amies transport medium plated on MRSASelect chromogenic agar and tryptic soy broth (27). The Cepheid GeneXpert detected 610 CFU/ml, corresponding to 58 CFU/swab, compared to 750 CFU/ml and 40 CFU/ml with the direct and enrichment cultures (27). Subsequent studies have reported the LoD for MRSA in the GeneXpert assay to be 109.4 CFU/assay (24) and 3,300 CFU/ml and 250 CFU/swab (26).…”

Duration of Contact Precautions for Acute-Care Settings

“…If positive, the specimen is assigned positive screen status. [99][100][101][102][103][104][105][106][107][108][109][110][111][112][113][114] This methodology (called "partial verification") introduces bias in favor of PCR. 115 For MRSA, Yam et al 99 tested 1,246 nares swab specimens with the Lightcycler MRSA Advanced (Roche Molecular Diagnostics, Basel Switzerland), a laboratory developed test (LDT) PCR and, in parallel, cultured all specimens.…”

Molecular diagnostics and genotyping of MRSA: an update

“…Rossney et al evaluated a commercial platform based on the BD GeneOhm principal for direct detection of MRSA in clinical samples, the GeneXpert MRSA assay (Cepheid Diagnostics, CA, USA) [35]. In this fully automatic, but low-throughput assay, cells are lysed using ultrasonics and DNA from samples is extracted, amplified and detected in separate chambers of single-use disposable cartridges, which contain freeze-dried beads with all reagents required for the real-time process.…”