Relationship between Sars-Cov-2 Viral Load, Antigen Positivity and Infectious Titer

ABSTRACT

Intro Performance of SARS-CoV-2 antigen tests is described relative to RT-PCR cycle threshold (Ct) values. However, Ct values may not be consistent between tests from different manufacturers, or even between runs. Here we correlate the Roche Elecsys SARS-CoV-2 Antigen assay results to a quantitative RT-PCR readout as a more reproducible measure of viral load. Additionally, we look at the relationship between the antigen test results, viral load and infectious titer. Method(s) Longitudinal nasopharyngeal swab samples from patients (N=452) with severe Covid-19 pneumonia collected between 03 April and 28 May 2020 in a randomized, double-blind, placebo controlled, multicenter study to evaluate the safety and efficacy of Tocilizumab (COVACTA), were assessed for SARS-CoV-2 viral load (RNA copies/ mL) and a qualitative and semi-quantitative readout of Elecsys SARS-CoV-2 Antigen assay. Viral culture experiments were performed to determine the infectious titer (TCID50/ mL) in a subset of samples. Agreement analysis was performed to compare the results of the assays. Please note that the current intended use of Elecsys SARS-CoV-2 Antigen assay is the qualitative detection of SARS-CoV-2 antigen. Finding(s) We observed high negative percent agreement between the Elecsys SARS-CoV-2 Antigen assay results and the RT-PCR results, while the positive percent agreement was only high in samples exceeding a certain viral load and at earlier time points from symptom onset. Infectious titer values and both the antigen assay semi-quantitative readout and the quantitative RT-PCR results correlated well. Positive percent agreement of RT-PCR and antigen results in relation to infectious titer was very high in both cases, while negative percent agreement was moderate to low. Conclusion(s) These data show that in patients with high viral load the Elecsys SARS-CoV-2 Antigen assay correlates well qualitatively and quantitatively with the presence of SARS-CoV-2 RNA and infectious virus as determined by RT-PCR and viral culture, respectively.Copyright © 2023