Rapid Quantification of SARS-CoV-2 Neutralising Antibodies Using Time-Resolved Fluorescence Immunoassay

The quantification of neutralising antibodies (NAb) for SARS-CoV-2 has become an important tool for monitoring protective immunity following infection or immunisation. In this study, we evaluated using World-Health-Organisation-standard immunoglobulin preparations, a novel point-of-care test that quantitates NAb by time-resolved fluorescent immunoassay. The assay provided robust data of binding antibody units (BAU) in 15 min that were well correlated with NAb values obtained by traditional in vitro neutralisation assay. The data also correlated well to spike-receptor-binding domain-binding antibodies over a broad range of plasma dilutions. The assay was extremely sensitive, able to detect positive samples after dilution 1:10,000 and over a wide range of BAU. Assay specificity was estimated at 96% using Pre-COVID-19 serum samples when applying a cut-off value of 47 BAU/mL, although readings of up to 100 BAU/mL could be considered borderline. This point-of-care diagnostic test is useful for rapid population screening and includes the use of capillary blood samples. Furthermore, it provides results for SARS-CoV-2 NAb in 15 min, which can inform immediate decisions regarding protective immunity levels and the need for continued COVID immunisations.