Low BAU/ml values with 4 of 5 SARS CoV-2 spike-specific monoclonal antibodies in the Roche Elecsys antibody assay

Background: The Abbott SARS-CoV-2 IgG Quant II assay and the Roche Elecsys double antigen sandwich (DAgS) immunoassay measure SARS-CoV-2 receptor binding domain (RBD)-specific antibodies in serum samples in different ways. The IgG Quant II assay uses an antigen in combination with a secondary antibody and the DAgS assay uses two antigens. The aim of the study was to investigate whether the assays give comparable results with monoclonal antibodies.Material and Methods: The immunoassays were tested with the RBD-specific human monoclonal antibodies (mAbs) casirivimab. imdevimab, CR3022, etesevimab and sotrovimab. The mAbs were tested at various concentrations in mu g/ml, alone or in combination and the relative light units (RLU) and binding antibody units (BAU)/ml were determined.Results: With 1 mu g/ml of casirivimab, imdevimab, CR3022 and etesevimab the Abbott IgG II Quant assay yielded between 65 and 158 BAU/ml and the Elecsys assay < 0.4 - 7.1 BAU/ml. In the DAgS assay, the addition of a second and a third mAb increased the BAU/ml values synergistically. With increasing concentrations of the mAb combinations in mu g/ml the Abbott IgG Quant II assay showed proportionate and the Elecsys DAgS assay disproportionate increases in BAU/ml. With 1 mu g/ml sotrovimab the Abbott assay gave 39 and the Elecsys assay 136 BAU/ml. The DAgS assay showed a high dose hook effect in the mu g/ml range.Conclusions: The secondary antibody-based and the DAgS-based SARS CoV-2 antibody assays gave very different results with 4 of 5 mAbs. This suggests that the two assays measure different binding characteristics. The ability of antibodies to cross-link multiple antigen-antibody complexes may contribute to the measurement signal in the DAgS assay.