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Xpert Ebola assay reduces time to EVD diagnosis
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The Xpert Ebola assay required nearly half the amount of time to detect Ebola virus disease in venous blood samples compared with a standard real-time PCR assay currently used in West Africa, according to recent data.
“Although conventional PCRs have high specificity and sensitivity, the time between sampling and obtaining results can be considerable, in particular in settings in which a laboratory with PCR capacity is not readily available,” Rafael Van den Bergh, PhD, a senior fellow in the operational research unit of Médecins Sans Frontières, and colleagues wrote.
The Xpert Ebola assay (Cepheid) received Emergency Use Authorization from the FDA in March for the in vitro detection of Ebola virus disease (EVD) in moderate and high complexity laboratories. The test is designed for GeneXpert systems, which have been used in Guinea for tuberculosis diagnoses. The system consists of a GeneXpert instrument, personal computer and disposable cartridges. The Xpert Ebola cartridge was designed to target the Ebola virus Zaire strain in the nucleocapsid protein (NP) and glycoprotein (GP) genes.
Van den Bergh and colleagues performed a cross-sectional study to assess the feasibility of the Xpert Ebola assay in an Ebola treatment center in Conakry, Guinea. The assay’s performance was compared with that of a conventional PCR test used for routine diagnostics.
The researchers tested 218 venous blood samples collected from 148 patients at the Ebola treatment center between May 2 and July 4. An inactivating agent was administered to the samples for 20 minutes, which is twice the recommended inactivation duration suggested by the manufacturer. The samples were then automatically processed and underwent reverse transcription, real-time PCR amplification.
The median run time of the GeneXpert instrument was 94 minutes. Ten tests were repeated due to power failures (n = 8), temperature failure and one invalid result. Including the repeated tests, Van den Bergh and colleagues estimated the Xpert Ebola assay produced results in half the amount of time it took for in-house PCR to yield results (163 minutes vs. 334 minutes; P < .0001).
Twenty-six positive samples were identified using the Xpert Ebola assay vs. 18 positive samples with conventional PCR. All eight additional samples identified with the Xpert Ebola assay had low viral loads ranging from 33 to 40.8 for the NP gene.
“In an outbreak setting, even one false-negative result, which would result in not identifying an EVD case, could initiate a new chain of transmission,” Van den Bergh and colleagues wrote. “Thus, the Xpert assay might represent a promising strategy toward rapid EVD diagnosis because its sensitivity is at least equivalent with that of conventional PCR.”
Similar results from a previous study that compared the Xpert assay with a conventional real-time PCR assay (RT-PCR; Trombley) were presented at IDWeek 2015. Twenty-two whole blood samples that tested positive for EVD using the Trombley also tested positive using the Xpert assay, yielding a sensitivity of 100% (95% CI, 84.6-100), according to researchers. They also found that 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 indicated that 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). 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) compared with the Trombley assay. The turnaround time of the Xpert assay was approximately 2.5 hours vs. 4 hours with the Trombley.
During the current study, all five laboratory technicians who utilized the Xpert Ebola assay said the test was overall easy to use; however, several concerns were noted regarding biosafety and logistics. The researchers wrote that technicians must have adequate working space, running air conditioning, a system to manage power failures and a safe way to transfer biologic material to the inactivation vial.
“Wider implementation of the Xpert Ebola Assay is recommended for facilities capable of supporting safe sampling and patient management if training of laboratory staff and standard operating procedures can be provided,” Van den Bergh and colleagues concluded. “Additional research into sensitivity and specificity of this test for patient samples is encouraged.”
Reference:
Pollock NR, et al. Abstract LB-1. Presented at: IDWeek; Oct. 7-11, 2015; San Diego.
Disclosure: The researchers report no relevant financial disclosures.
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