Validation of a focus reduction neutralization test (FRNT) to rapidly titrate human SARS-CoV-2 neutralizing antibodies by using the CTL Immunospot S6 universal analyzer

In this paper we describe the validation of a focus reduction neutralization test (FRNT) to quantitate human SARS-CoV-2 neutralizing antibodies by using the CTL Immunospot S6 Universal Analyzer. We employed a previously published protocol and compared its performances to a well-established and traditional serum-neutralization assay (SN). To assess diagnostic sensitivity, a total number of 201 human sera positive by SN for SARS-CoV-2 NAbs were processed: 196/201 tested positive by FRNT50 (97.51 %). A diagnostic specificity of 100 % was obtained by evaluating 206 negative serum samples. Repeatability of the test was evaluated by determining the intra and inter-assay coefficient of variation (CV). A standard deviation of 0.83 and a CV of 13 % were evidenced demonstrating an acceptable reproducibility of the assay. Moreover, a Cohen's Kappa of 0.975 was obtained proving an extremely high level of agreement between the FRNT protocol and the SN. Despite an acceptable correlation between methods (p