Authors:

Abstract

Introduction: As the daily number of patients diagnosed with SARS-CoV-2 infection by PCR increases, the necessity to identify truly infectious cases becomes more significant. We aimed to identify a cut-off Ct value of the COVID-19 RT-PCR assay for likely infectivity by assessing the COVID-19 IgG status and investigating the utility of the Rapid Antigen Test (RAT) in identifying infectious cases among asymptomatic individuals.

Methods: Nasopharyngeal/throat swabs were simultaneously taken for COVID-19 RT-PCR and RAT from 552 asymptomatic individuals at De Soysa Maternity Hospital, Colombo, from 23^rd of November to 19^th of December 2020. In addition, SARS-CoV-2 IgG (against nucleoprotein) status in PCR positive individuals was evaluated when simultaneously taken sera was available.

Results: COVID-19 RT-PCR positive rate among asymptomatic individuals was 14.3% (n=79).  The overall sensitivity of RAT was 30.4% but increased to 73.9% when Ct values below 25 were considered. The COVID-19 IgG response was evaluated in 37 PCR positive subjects and the overall seropositivity was 40.5%.  The optimal Ct thresholds for discrimination of COVID-19 IgG status were 30.5 and 30.29 for the E and S gene respectively. There was a significant positive correlation between Ct values of the E gene and IgG ratio values (r=0.345, p<0.05). The Ct thresholds for RAT positivity were 26.5 and 26.06 for E and S genes respectively, with a significant negative correlation (p<0.001).

Conclusion: We conclude that it is possible to define a cut off Ct value in SARS-CoV-2 PCR (with some error margin for practical purposes) for likely non-infectivity. It was also deduced that positive COVID-19 rapid antigen result seems to be more predictive of infectivity in comparison to positive PCR result.

PCR result.

Keywords: COVID-19, Ct value, PCR assay, Rapid antigen test, SARS-CoV-2