Application of digital PCR to determine the reliability of Xpert Xpress SARS-CoV-2 assay with envelope (E) gene negative and nucleocapsid (N2) gene positive results.

This study used digital polymerase chain reaction (dPCR) to determine whether envelope (E) gene-negative and nucleocapsid (N2) gene-positive (E-N+) results obtained with the Cepheid Xpert Xpress SARS-CoV-2 assay are reliable. Using droplet digital PCR results as a reference, 18 of 22 E-N+ samples with a low viral load (81.8%) were identified as true positives.

Evaluation on testing of deep throat saliva and lower respiratory tract specimens with Xpert Xpress SARS-CoV-2 assay.

BACKGROUND: Xpert® Xpress SARS-CoV-2 assay is only validated on nasopharyngeal specimens for detection of SARS-CoV-2. Other specimen types such as deep throat saliva (DTS), also known as posterior oropharyngeal saliva and lower-respiratorytract specimens (LRT) including sputum, tracheal aspirate and bronchoalveolar lavage are not validated. These non-validated specimen types, however, do have significant diagnostic value. OBJECTIVE: Evaluate the performance of Xpert Xpress SARS-CoV-2 assay for detection of SARS-CoV-2 from DTS and LRT specimens. METHODS: 162 specimens from 158 patients with suspected COVID-19 disease were tested with Xpert Xpress SARS-CoV-2 assay. These included 120 DTS and 42 LRT specimens i.e. 35 sputum, 6 tracheal aspirate and one bronchoalveolar lavage. Results were compared to those by the TIB-Molbiol LightMix® SarbecoV E-gene assay. RESULTS: Xpert Xpress SARS-CoV-2 assay has satisfactory performance when compared with reference method. The positive percent agreement (PPA) of DTS and LRT specimens were 98.86 % & 100 % respectively while the negative percent agreement (NPA) was 100 % for both DTS and LRT specimens. CONCLUSIONS: This study demonstrated with appropriate sample pre-treatment, Xpert Xpress SARS-CoV-2 assay can be used to test on non-validated specimen types including DTS & LRT specimens.