Background: This study was planned to investigate how the positivization time of a SARS-CoV-2 rapid antigen self-test may correlate with SARS-CoV-2 nucleocapsid (N) antigen concentration measured with a quantitative laboratory-based immunoassay. Methods: Paired nasopharyngeal (healthcare-collected) and nasal (self-collected) samples were taken from patients undergoing routine SARS-CoV-2 testing. The concentration of SARS-CoV-2 antigen nucleocapsid (N) was assayed with Liaison SARS-CoV-2 Antigen test, whilst the time of positivization of COVID-VIRO ALL rapid diagnostic test (RDT) was concomitantly measured and then compared SARS-CoV-2 viral load measured with Liaison SARS-CoV-2 Antigen test and expressed as Median Tissue Culture Infectious Dose (TCID50)/mL. Results: The study sample consisted of 32 paired specimens which tested positive with COVID-VIRO ALL IN RDT and had SARS-CoV-2 N protein concentration measured with Liaison SARS-CoV-2 Antigen test. A highly significant correlation was found between SARS-CoV-2 viral antigen concentration and RDT positivization time (r=-0.64; 95%CI, -0.81 to -0.38; p<0.001). At the >1500 TCID50/mL threshold of the Liaison SARS-CoV-2 Antigen test, the positivization time of the COVID-VIRO ALL IN RDT displayed high accuracy (93.7%). A positivization time <42 sec enabled to identify patients with high SARS-CoV-2 antigen concentration (i.e., >1500 TCID50/mL) with 91.3% negative and 100% positive predictive values. Conclusion: Self-testing using COVID-VIRO ALL IN RDT could be reliably used for garnering valuable information on the actual SARS-CoV-2 viral antigen concentration in respiratory samples.
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