Comparison of Six Immunoassays for Assaying Levels of Immunoglobulin G against the Nucleocapsid and Spike Proteins of SARS-CoV-2

Introduction: Assay kits for detection of Immunoglobulin G (IgG) against the nucleocapsid protein (anti-nucleocapsid IgG) and spike proteins (anti-spike IgG) of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) were commercially provided by several manufacturers. These assay kits should be verified by measuring the same sample. Aim: To compare the diagnostic value of three Coronavirus Disease-2019 (COVID-19) kits in evaluating six immunoassays developed by three manufacturers (Abbott, Euglena, and Roche) to detect anti-nucleocapsid IgG and anti-spike IgG. Materials and Methods: The present study was an observational cross-sectional study conducted from June 2020 to December 2020. Antibody titers for anti-nucleocapsid IgG and anti-spike IgG among 429 Healthcare Workers (HCWs) in a Tone Central Hospital, Japan where a nosocomial infection of the COVID-19 occurred were measured by six immunoassays with kits developed by three different manufacturers. The sensitivity and specificity of each kit was compared to real-time Reverse Transcription-Polymerase Chain Reaction (RT-qPCR). Results: Six of the HCWs tested positive for SARS-CoV-2 via RT-qPCR, and the rest tested negative. The severity of COVID-19 among these six HCWs ranged from mild to moderate. The sensitivity and specificity values against RT-qPCR were, 100% and 99.5% for Abbott, 83.3% and 100% for Euglena, and 100% and 100% for Roche when using the nucleocapsid protein assay and 100% and 99.8% for Abbott, 100% and 100% for Euglena, and 100% and 100% for Roche when using the spike protein assay kit. Conclusion: The commercial kits provided by three manufacturers reflected the immune status of individuals. There were no major differences in the performance of these test kits. Discordant results with the antibody titer for anti-nucleocapsid IgG and anti-spike IgG were detected by using assay kits provided by Abbott and Euglena. To evaluate the past history of COVID-19, it should be noted that the single measurement of anit-nucleocapsid IgG or anti-spike IgG could not exclude false negative or positives.