COVID-19 diagnosis and study of serum SARS-CoV-2 specific IgA, IgM and IgG by a quantitative and sensitive immunoassay

Background The current pandemic of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has caused a great loss in lives and economy. Detecting viral RNAs on nasopharyngeal and throat swabs is the standard approach for SARS-CoV-2 diagnosis with variable success. Currently, there are only a few studies describing the serological diagnostic methods that involve the detection of SARS-CoV-2-specific IgM and IgG. Here, we aimed to develop a more quantitative and sensitive serological test for COVID-19 diagnosis, monitoring and clinical investigation, based on the detection of antigen-specific IgA as well as IgM and IgG in blood in response to SARS-CoV-2 infection. Methods In this investigation, we report the development of a set of validated diagnostic kits for detecting serum IgA, IgM, and IgG specific to SARS-CoV-2 nucleocapsid protein (NP) and receptor-binding domain (RBD) of the spike protein by chemi-luminescence immuno-analysis. The kits were tested with a cohort of 216 sera from 87 laboratory-confirmed COVID-19 patients, and 483 sera from SARS-CoV-2 negative or healthy individuals as negative controls. A standard receiver operating characteristic (ROC) analysis was conducted to evaluate the diagnostic accuracy. Using the kits, serum levels of IgA, IgM, and IgG were analyzed, in response to SARS-CoV-2 infection and COVID-19 pathogenesis. Findings The diagnostic kits based on the RBD antigen outperformed those based on the NP. RBD-specific IgA, IgM, and IgG detection kits showed sensitivities of 98.6%, 96.8%, and 96.8%, and specificities of 98.1%, 92.3%, and 99.8%, respectively. In addition, using purified RBD-specific immunoglobulins from a serum pool of COVID-19 patients as standards, the serum concentrations of RBD-specific IgA, IgM, and IgG proteins were determined. The concentrations varied widely among different patients. Median concentration of IgA and IgM reached peaks at 16-20 days after illness onset at 8.84 μg/mL and 7.25 μg/mL, respectively, while median concentration of IgG peaked during 21-25 days after illness onset at 16.47 μg/mL. Furthermore, the serum IgA level positively correlates with COVID-19 severity. Interpretation Our immunoassay of measuring SARS-CoV-2 specific antibodies IgA, IgM, and IgG in serum provides a better serological testing with improved sensitivity and specificity. Data of IgA, IgM, and IgG responses in blood of COVID-19 patients may provide novel insight for the monitoring and treatments of COVID-19. The kits are also suitable for epidemiological studies and vaccine validations.