Details for BIL Poster GeneXpert ESCMID 2008

Name:BIL Poster GeneXpert ESCMID 2008

Objectives: Evaluation in a multi-centre setting of the performance of a rapid commercial molecular technique (Cepheid’s Xpert MRSA assay) for the detection of MRSA in surveillance samples of high-risk patients in comparison to standard culture technique.

Methods: High-risk patients (n=236) were sampled from April till June 2007 in 5 Belgian hospitals. Separate nasal, throat and perineal swabs were collected using the Cepheid collection device (Double Copan Swab). In addition, swabs were taken according to each specific hospital procedure.

One part of the double Copan nose, throat and perineum swabs was pooled and vortexed in the Elution Reagent and then transferred to the Xpert MRSA cartridge. Molecular testing was performed on the GeneXpert according manufacturer’s instructions. The system combines extraction, real-time PCR and detection in about one hour.

The other part of the double swabs was pooled, frozen and sent to 1 laboratory that performed the reference culture, using TSB with 5% NaCl and subculture after 24 hours on a chromogenic agar plate.

The own procedure swabs (nose, throat and perineum) were cultured in-house, also using selective enrichment and chromogenic agar according to local procedures.

Results: 27 Samples (11,4%) showed inhibition on the GeneXpert and could not be included in the analysis.

The Xpert MRSA assay identified 97.4% of the specimens positive for MRSA and 86.5% of the negative specimens. For the samples tested, the Positive Predictive Value was 62.3% and the Negative Predictive Value was 99.3%.

The PPV of the GeneXpert results versus the Reference culture and versus the In-house culture method differed significantly (62.3% versus 80.3%). Freezing and thawing might have been injurious to the viability of MRSA. For almost all hospitals, the PPV decreased significantly when performed with the Reference Culture method.

Conclusion: The Cepheid’s Xpert MRSA assay proves to be an efficient test to rule out MRSA (NPV=99.3%). The test provides rapid results (about one hour) which might be helpful in prevention and control of MRSA in hospitals. However, because of a high rate of ‘invalids' and a low PPV, a culture method is still needed to confirm a positive result.