Analytical and clinical performances of a SARS-CoV-2 S-RBD IgG assay: comparison with neutralization titers

Background: SARS-CoV-2 serology presents an important role in understanding the virus epidemiology, in vaccine prioritization strategies and in convalescent plasma therapy. Immunoassays performances have to be accurately evaluated and correlated with neutralizing antibodies to be used as a surrogate measure of neutralizing activity. We investigate the analytical and clinical performance of a SARS-CoV-2 RBD IgG assay, automated on a high throughput platform, and the correlation of the antibodies (Ab) levels with the plaque reduction neutralization (PRNT50) Ab titers. Methods: A series of 546 samples were evaluated by SARS-CoV-2 RBD IgG assay (Snibe diagnostics), including 171 negative and 168 positive SARS-CoV-2 subjects and a further group of 207 subjects of the COVID-19 family clusters follow-up cohort. Results: Assay precision was acceptable at low and medium levels; linearity was excellent in all the measurement range. Considering specimens collected after 14 days post symptoms onset, overall sensitivity and specificity were 99.0% and 92.5%, respectively. A total of 281 leftover samples results of the PRNT50 test were available. An elevated correlation was obtained between the SARS-CoV-2 RBD IgG assay and the PRNT50 titer at univariate (rho = 0.689) and multivariate (rho = 0.712) analyses. Conclusions: SARS-CoV-2 S-RBD IgG assay achieves elevated analytical and clinical performances, and a strong correlation with sera neutralization activity.