Automated Ebola Blood Test Performs Well in Field Evaluation

The prolonged and devastating outbreak of Ebolavirus disease (EVD) in West Africa has exposed a need for improved diagnostic methods to reduce result turnaround times and increase access to testing without sacrificing test accuracy.

The diagnosis of EVD has relied primarily on real-time reverse transcription polymerase chain reaction (RT-PCR) performed in field biocontainment laboratories on submitted venous whole blood (WB) or buccal swab (BS) specimens. Conventional RT-PCR-based testing is cumbersome, requiring multiple processing steps from sample lysis through nucleic acid extraction to the RT-PCR step itself.

An international team of scientists led by those at Public Health England (Porton Down, UK) tested 218 WB samples collected through venipuncture from patients with suspected or confirmed EVD, as well as 71 BS samples collected as part of a national postmortem screening program between March 7 and July 20, 2015. EDTA-WB and BS specimens were tested with Xpert (targets: glycoprotein [GP] and nucleoprotein [NP] genes) and Trombley (target: NP gene) assays in parallel.

The GeneXpert Ebola assay was performed using the GeneXpert Ebola Assay kit REBOLA-50 (Cepheid; Sunnyvale, CA, USA). All steps prior to loading test cartridges onto the Xpert instrument platform were carried out in flexible film isolators (FFIs). All samples were incubated in sample reagent for 20 minutes at room temperature, and then transferred to Xpert Ebola test cartridges, taking care not to introduce air bubbles. Cartridges were loaded onto a Cepheid’s GeneXpert XVI platform.

After excluding a few samples that gave Xpert results that were reported as "invalid" or "error," 22 out of 22 Trombley-positive WB samples were Xpert-positive (sensitivity of 100%), and 181 out of 189 Trombley-negative WB samples were Xpert-negative (specificity of 95.8%). In 64 BS samples with valid results for both Trombley and Xpert, the sensitivity and specificity of the Xpert assay were both 100%. For blood samples with enough volume for two tests, the investigators also compared sampling blood using a pipette versus a swab and found that 78 of 79 paired results were concordant, suggesting that collection of the same volume of fingerstick blood using a swab (which is potentially easier to do in the field) may also provide accurate results when using the GeneXpert assay.

The authors concluded that their findings suggest that the use of the Xpert Ebola assay could facilitate expanded access to Ebolavirus testing. Their data indicate that the Xpert Ebola assay has excellent performance in a field laboratory setting using both WB and BS specimens and thus provides the opportunity for highly accurate, rapid sample-to-answer diagnosis of EVD. The study was published on March 29, 2016, in the journal Public Library of Science Medicine.

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