RELATIONSHIP BETWEEN SARS-COV-2 VIRAL LOAD, ANTIGEN POSITIVITY AND INFECTIOUS TITER

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. METHODS: 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. FINDINGS: 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: 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.