Rapid Diagnostic Assay for Ebola Field Tested

Current diagnostic testing is performed by using PCR of ribonucleic acid (RNA) extracted from venous blood samples or swab samples. 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.

Scientists at the Médecins Sans Frontières (MSF; Brussels, Belgium) and their colleagues obtained blood samples at the same time as the convalescent-phase plasma trial at an Ebola Treatment Center (ETC). A total of 218 samples from 148 persons were collected during diagnosis, treatment, and convalescence of patients. In the high-risk area, 100-μL samples from each EDTA tube were directly transferred by using an automatic pipette with filter tips.

The Xpert Ebola assay (Cepheid Inc.; Sunnyvale, CA, USA) used in the study is fully automated and cartridge based (closed system) and includes automated controls for interference with the PCR and adequacy of sample input. This assay can be used with the Cepheid GeneXpert System, which is widely used for rapid detection of tuberculosis and rifampin resistance in decentralized settings. The only manual step is inactivation of the blood sample and transfer to the Cepheid cartridge; sample processing (RNA extraction), reverse transcription, real-time PCR amplification, and detection of TaqMan probes are then performed automatically.

Of 218 samples tested, the Xpert Ebola Assay identified 26 (12%) positive samples: 8 (5%) of 147 at initial diagnosis, 12 (100%) of 12 after transfusion and six (86%) of seven at convalescence. The routine laboratory identified 18 (69%) of the 26 positive samples identified by the Xpert Ebola Assay. No discordance was observed for diagnostic samples and no samples identified as negative in the Xpert Ebola Assay were identified as positive by the routine laboratory. The eight samples identified as positive in the Xpert Ebola Assay and as negative by the routine laboratory were five samples obtained during convalescence and three samples obtained after transfusion, had low viral loads and were obtained from four patients.

The authors concluded that that implementation of the Xpert Ebola Assay under programmatic conditions in an MSF ETC in Guinea is feasible and represents a major decrease in time required to obtain results and a possible increase in sensitivity compared with routine diagnostic procedures that use PCR in this setting. 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. The study was published in the February 2016 issue of the journal Emerging Infectious Diseases.

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Médecins Sans Frontières 
Cepheid Inc.