Performance Comparison of a Flow Cytometry-based and Two Commercial Chemiluminescent Immunoassays for Detection and Quantification of Antibodies Binding to SARS-CoV-2 Spike Protein

The performance of a laboratory-developed quantitative IgG/IgA flow cytometry-based immunoassay (FCI) using Jurkat T cells stably expressing full-length native S protein was compared against Elecsys(R) electrochemiluminiscent (ECLIA) Anti-SARS-CoV-2 S (Roche Diagnostics, Pleasanton, CA, USA), and LIAISON(R) SARS-CoV-2 TrimericS IgG chemiluminiscent assay (CLIA) (Diasorin S.p.a, Saluggia, IT) for detection and quantitation of SARS-CoV-2-specific antibodies. A total of 225 serum/plasma specimens from 120 acute or convalescent COVID-19 individuals were included. Overall, IgG/IgA-FCI yielded the highest number of positives (n=179), followed by IgA-FCI (n=177), Roche ECLIA (n=175), IgG-FCI (n=172) and Diasorin CLIA (n=154). Positive percent agreement between FCI and compared immunoassays was highest for Roche ECLIA, ranging from 96.1% (IgG/IgA-FCI) to 97.7% (IgG-FCI), whereas negative percent agreement was higher between FCI and Diasosin CLIA, regardless of antibody isotype. A strong correlation (Rho:0.6-0.8) was found between IgG-FCI or IgA-FCI levels and antibodies quantified by Roche ECLIA and Diasorin CLIA. The trajectory of antibody levels delineated by the different immunoassays in 22 of patients with sequential specimens ([≥]3) was frequently discordant, with the exception of IgG and IgA determined by FCI assay and to a lesser extent antibodies quantified by Roche ECLIA and Diasorin CLIA. The data suggest that FCI may outperform Roche ECLIA and Diasorin CLIA in terms of clinical sensitivity for serological diagnosis of SARS-CoV-2 infection.