Comparative study of a new commercial recombinant line assay and two immunoassays for detection of SARS-CoV-2 IgG antibodies in PCR-confirmed outpatients

Commercially available immunoassays have been developed for detection of antibodies against SARS-CoV-2. However, equivocal and discrepant results between different immunoassay can occur requiring further assessment by confirmatory tests.We investigated the new commercial line assay recomLine SARS-CoV-2 IgG (Mikrogen, containing the antigens S1, receptor-binding domain of the spike protein, and nucleocapsid protein (NP) of SARS-CoV-2) within a collection of well characterized serum samples from COVID-19 outpatients (n=49) and SARS-CoV-2-PCR-positive asymptomatic contact persons (n=6) in comparison to two commercial immunoassays, the S1 antigen based Anti-SARS-CoV-2-ELISA IgG by Euroimmun and the NP based Elecsys® Anti-SARS-CoV-2 by Roche.The recomLine assay was positive in all samples which had an equivocal or positive result for SARS-CoV-2 antibodies in at least one of the two immunoassays. It showed high agreement with the overall results of the immunoassays (94.5% [Cohen’s kappa = 0.85] and 92.7% [Cohen’s kappa 0.81] to the ELISA by Euroimmun and the assay by Roche, respectively). In addition, high agreement of the reactivity to the specific antigens S1 and NP in the recomLine assays compared to the results of the S1 based ELISA and NP based Elecsys® assay, was found (90.9% [Cohen’s kappa 0.78] and 96.4% [Cohen’s kappa 0.91] for S1 and NP, respectively).The new recomLine SARS-CoV-2 IgG assay may be used as an additional tool for investigation of equivocal or discrepant results of Anti-SARS-CoV-2 immunoassays and for antigen-specific detection of SARS-CoV-2 IgG antibodies.