Multitarget Assay Improves MRSA Screening

A molecular screening assay that targets three genes of methicillin-resistant Staphylococcus aureus (MRSA) is suggested as more accurate than commercial assays that target a single gene.

The detection of the staphylococcal cassette chromosome mec (SCCmec) gene is a common strategy used for screening for MRSA, but when used alone, it may not be specific enough and therefore a multitarget assay should improve detection.

Scientists at the Sir Mortimer B. Davis (SMBD)-Jewish General Hospital (Montreal, QC, Canada), screened a total of 675 clinical isolates of which more than 95% were nasal swabs, that were submitted for MRSA screening between January 2012 and March 2012. Coagulase and antimicrobial susceptibility testing was performed using enrichment broth and inoculated onto blood agar plates.

MRSA screening was performed with an in-house polymerase chain reaction (PCR) with primers targeting the methicillin resistance gene (mecA), the staphylococcal nuclease gene (nuc), and the coagulase gene (coa). This assay was compared with the BD GeneOhm MRSA PCR ACP Assay Amplification Kit (GeneOhm Sciences Canada, Inc.; Québec City, QC, Canada). The test characteristics of the commercial MRSA PCR and in-house MRSA PCR assays were calculated by comparison to the gold standard of culture results. The in-house assays were performed on the LC480 (Roche; Indianapolis, IN, USA) and the SLAN (Daan Diagnostics Ltd.; Burnaby, BC, Canada) real-time quantitative PCR instruments.

From the 675 swabs tested, 20 tested positive for MRSA by culture and by both PCR assays. From the remaining 655 samples, 30 were positive by the commercial MRSA PCR, but were negative by both the in-house real-time PCR assay and culture, and five other samples were positive by the in-house PCR assay, but were negative by commercial MRSA PCR and by culture. The performance characteristics of the in-house assay were significantly improved compared with the commercial assay, leading to an increase of positive predictive value by 40%. The sensitivity of the in-house assay was 100% and the specificity was 99.2% compared to the bacterial cultures.

The authors concluded that their alternative in-house PCR assay had a better positive predictive value and overall clinical performance than the commercial kit, but at the cost of increased turnaround time due to the need for overnight incubation. The assay is suitable for a wide range of real-time PCR platforms used in clinical microbiological laboratories. To decrease the potential for false-positive calling, the assay is being optimized combining delta crossing points (Cp) strategy and a SCCmec PCR assay. The study was published on July 16, 2013, in the Journal of Molecular Diagnostics.

Related Links:
Sir Mortimer B. Davis-Jewish General Hospital
GeneOhm Sciences Canada
Daan Diagnostics