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Xpert Ebola test has 'excellent' performance
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SAN DIEGO — Recent findings suggest the Xpert Ebola assay had excellent performance on whole blood and buccal swab specimens submitted for routine testing for Ebola virus disease.
“The Xpert platform has already demonstrated feasibility for use in resource-limited settings for diagnosis of TB, though primarily at the district level, partly because of the need for a continuous and stable power supply,” Nira R. Pollock, MD, PhD, associate medical director of the Infectious Diseases Diagnostic Laboratory at Boston Children's Hospital, said during her presentation.
Nira R. Pollock
In March, the Xpert Ebola assay (Cepheid), which runs on GeneXpert systems, received Emergency Use Authorization from the FDA for the in vitro detection of Ebola virus in moderate and high complexity laboratories.
Pollock and colleagues from Partners In Health and Public Health England (PHE) evaluated the performance of the Xpert Ebola assay on 211 clinical venipuncture whole blood specimens and 64 buccal swab specimens at a PHE field biocontainment laboratory in Port Loko, Sierra Leone, comparing it with that of a conventional real-time PCR assay (RT-PCR; Trombley), which was already in use in the laboratory. The Xpert test, which integrates nucleic acid extraction and amplification, targets both the glycoprotein (GP) and nucleoprotein (NP) genes belonging to the Zaire ebolavirus. The Trombley test, which utilizes viral RNA extracted from plasma or buccal swab samples, targets only the NP gene.
A positive result from the Trombley is determined by a cycle threshold (Ct) value of 40 or less. “For Xpert, all interpretation is done by the system — there is no operator interpretation,” Pollock said.
In addition, a subset of whole blood samples was tested in duplicate on the Xpert assay to compare blood sampling methods (pipette vs. swab).
All 22 whole blood samples that tested positive for Ebola virus disease (EVD) using the Trombley also tested positive using the Xpert assay, yielding a sensitivity of 100% (95% CI, 84.6-100). Pollock said 181 out of 189 whole blood samples that tested negative by Trombley tested negative using the Xpert assay, yielding a specificity of 95.8% (95% CI, 91.8-98.2). Further analysis suggested seven of the eight discrepant samples were submissions from previously Trombley-positive EVD patients, giving the Xpert assay a revised specificity of 99.5% (95% CI, 97-100).
“What we learned is that these discrepant samples had come from patients who were already known to have Ebola virus disease,” Pollock said.
In buccal swab samples, the Xpert assay had a sensitivity of 100% (95% CI, 83.2-100) and a specificity of 100% (95% CI, 92-100) vs. the Trombley assay.
Both positive and negative Xpert results for whole blood sampled by pipette vs. swab were concordant for 78 out of 79 (98.7%) samples, with comparable Ct values for positive samples.
“We saw excellent performance of the Xpert Ebola assay on both whole blood and buccal swab samples in a field laboratory setting as compared to a well-vetted benchmark,” Pollock said, adding that the Xpert is potentially more sensitive than the Trombley using samples with a low viral load, as evidenced by the discrepant analysis of the eight whole blood samples.
Additionally, the Xpert assay was easier to use than the standard RT-PCR, Pollock said, with a turnaround time of approximately 2.5 hours vs. 4 hours with the Trombley.
One potential drawback associated with the Xpert assay is the temperature needed to store its components — including buffers and cartridges — which ranges between 2 C and 28 C. According to Pollock, the temperature range in the laboratory during the study was 23 C to 31.7 C (mean, 27.9 C). The need for an uninterrupted power supply and the level of expertise required to set up, operate and maintain the test platform also are potential barriers to its use in resource-limited settings. Moreover, Pollock described ongoing controversy regarding whether the sample reagent is capable of fully deactivating live Zaire ebolavirus in samples with extremely high viral loads.
“I think this is a question that must be considered in order to determine the full range of potential settings for assay deployment in this region,” Pollock said. – by John Schoen
Reference:
Pollock NR, et al. Abstract LB-1. Presented at: IDWeek; Oct. 7-11, 2015; San Diego.
Disclosure: Pollock reports no relevant financial disclosures.