Comparing the performance of two commercial assays for detection of SARS-CoV-2 in samples with low viral load

Abstract

be safely discharged from isolation. We recently reviewed the Ct values from 24 consecutive nasopharyngeal swab samples received between June and July 2020 by the laboratory with presumptive positive SARS-CoV-2 RNA by cobas ® (a positive result for the E gene target but a negative result for the ORF1a gene target). These are specimens with a borderline viral titre close to the assay’s limit of detection (LOD) (100 copies/mL) (3) as indicated by their high Ct values. These samples were re-tested within the same day using the Xpert ® Xpress SARS-Cov-2 assay. This assay detects the N2 and E gene targets. Similar to the reporting algorithm of the cobas ® test, samples with only detectable E gene by the Xpert ® assay are considered as presumptive positive with recommendation for re-testing (Xpert ® Xpress SARS-Cov-2 assay package insert). The LOD of Xpert ® assay was determined by Wolters et al. to be 8.26 copies/mL (4). 24 presumptive positive samples by cobas test, 17 E and N2 gene Ct value for E gene target Ct value for gene for both E and N2 gene they were tested using the Xpert ® assay. Close agreement between the Ct values of E gene target of the two assays observed, with a high standard Stochastic effect, which is a prominent observation due to random variations when examining low