Accurate and rapid diagnostic tools are needed for management of the ongoing coronavirus disease 2019 (COVID-19) pandemic. Antibody tests enable detection of individuals past the initial phase of infection and help examine vaccine responses. The major targets of human antibody response in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are the spike glycoprotein (SP) and nucleocapsid protein (NP). We have developed a rapid homogenous approach for antibody detection termed LFRET (protein L-based time-resolved Forster resonance energy transfer immunoassay). In LFRET, fluorophore-labeled protein L and antigen are brought to close proximity by antigen-specific patient immunoglobulins of any isotype, resulting in TR-FRET signal. We set up LFRET assays for antibodies against SP and NP and evaluated their diagnostic performance using a panel of 77 serum/plasma samples from 44 individuals with COVID-19 and 52 negative controls. Moreover, using a previously described SP and a novel NP construct, we set up enzyme linked immunosorbent assays (ELISAs) for antibodies against SARS-CoV-2 SP and NP. We then compared the LFRET assays with these ELISAs and with a SARS-CoV-2 microneutralization test (MNT). We found the LFRET assays to parallel ELISAs in sensitivity (90-95% vs. 90-100%) and specificity (100% vs. 94-100%). In identifying individuals with or without a detectable neutralizing antibody response, LFRET outperformed ELISA in specificity (91-96% vs. 82-87%), while demonstrating an equal sensitivity (98%). In conclusion, this study demonstrates the applicability of LFRET, a 10-min "mix and read" assay, to detection of SARS-CoV-2 antibodies.
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