Details for BIL Poster GeneXpert Eswab ESCMID 2009

Name:BIL Poster GeneXpert Eswab ESCMID 2009


The Xpert MRSA® assay (Cepheid), which runs exclusively on the GeneXpert® system (Cepheid), is a FDA approved molecular test to screen for MRSA. It has previously been validated on nose, throat and perineum samples, taken by a double Copan swab® (Copan). This study evaluates, in a multi-centre setting, the use of pooled eSwab® liquid transport medium (Copan) from nose, throat and perineum (NTP) as input sample for the Xpert MRSA® assay in comparison to standard culture technique from the same medium.


High-risk patients (n=159) were sampled from July until September 2008 in 5 Belgian hospitals. Separate nasal, throat and perineum swabs were collected using the eSwab®.

 Four hundred microL of each eSwab® liquid medium from a NTP set was pooled to a final volume of 1200 microL. For the molecular test using the Xpert MRSA® assay, 150 microL of pooled sample was added to the lysis buffer provided in the kit. Further testing was performed according to the manufacturer’s instructions. Another 500 microL of pooled sample was transferred to 4 mL of TSB and incubated for 18-24h at 35°C. Ten microL of this enriched culture was transferred to a MRSA-ID® plate (bioMérieux) and screened for the presence of MRSA after 24 and 48 hours.


Twenty-nine (18,2%) samples were MRSA positive on culture. Of these, 28 (96,6%) were Xpert MRSA positive, while 1 (3.4%) tested negative. Of the 130 culture negative samples, Xpert MRSA was negative in 125 (96,2%) but positive in 5 (3,8%) samples. Sensitivity and specificity of the Xpert MRSA assay was 96,6% and 96,2%, respectively. The positive predictive value (PPV) was 84,8% and the negative predictive value (NPV) was 99,2%. No invalid results were observed for the Xpert MRSA® assay.


The results of Xpert MRSA® on eSwab® liquid transport medium pooled from NTP are comparable to the results previously obtained with the double Copan swab ®, but fewer invalid results were obtained with eSwab®. The high NPV (99,2%) makes it suitable to rule out MRSA. However, due to the lower PPV (84.8%), positive Xpert MRSA® results need confirmation by culture. For culture, the same eSwab fluid can be used, reducing the risk of sampling bias.

Pooled eSwab® liquid medium from NTP is an adequate matrix for rapid MRSA screening by the Xpert MRSA® assay with culture confirmation possibility without extra sampling.