Evaluation of Nucleocapsid and Spike Protein-Based Enzyme-Linked Immunosorbent Assays for Detecting Antibodies against SARS-CoV-2

Abstract

At present, PCR-based nucleic acid detection cannot meet the demands for coronavirus infectious disease (COVID-19) diagnosis. Two hundred fourteen confirmed COVID-19 patients who were hospitalized in the General Hospital of Central Theater Command of the People's Liberation Army between 18 January and 26 February 2020 were recruited. Two enzyme-linked immunosorbent assay (ELISA) kits based on recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (rN) and spike protein (rS) were used for detecting IgM and IgG antibodies, and their diagnostic feasibility was evaluated. Among the 214 patients, 146 (68.2%) and 150 (70.1%) were successfully diagnosed with the rN-based IgM and IgG ELISAs, respectively; 165 (77.1%) and 159 (74.3%) were successfully diagnosed with the rS-based IgM and IgG ELISAs, respectively. The positive rates of the rN-based and rS-based ELISAs for antibody (IgM and/or IgG) detection were 80.4% and 82.2%, respectively. The sensitivity of the rS-based ELISA for IgM detection was significantly higher than that of the rN-based ELISA. We observed an increase in the positive rate for IgM and IgG with an increasing number of days post-disease onset (d.p.o.), but the positive rate of IgM dropped after 35 d.p.o. The positive rate of rN-based and rS-based IgM and IgG ELISAs was less than 60% during the early stage of the illness, 0 to 10 d.p.o., and that of IgM and IgG was obviously increased after 10 d.p.o. ELISA has a high sensitivity, especially for the detection of serum samples from patients after 10 d.p.o., so it could be an important supplementary method for COVID-19 diagnosis.

Keywords: COVID-19 diagnosis; ELISA; IgG; IgM; antibody; nucleocapsid protein; spike protein.

FIG 1

Comparison of positive rates of antibodies detected by rN-based ELISA and rS-based ELISA. IgM means a positive result of IgM antibody detection by ELISA, IgG means a positive result of IgG antibody detection by ELISA, and IgM and/or IgG means at least one of them was positive by IgM and IgG ELISA. Results were compared by chi-square tests.

FIG 2

Dynamic trend of the positive rate of IgM and IgG in serum of patients at different stages of disease. Patients were divided into seven groups of 0 to 5, 6 to 10, 11 to 15, 16 to 20, 21 to 30, 31 to 35, and >35 days post-disease onset. (a) Dynamic trend of antibody-positive rate detected by rN-based ELISA. (b) Dynamic trend of antibody-positive rate detected by rS-based ELISA.