Designing an ELISA Method for Measurement of Human IgG and IgM Antibodies against SARS-CoV-2

Background and purpose: Coronavirus disease 2019 (COVID-19) is a respiratory disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In this study, an indirect ELISA method was designed to measure the human IgM and IgG antibodies against SARS-CoV-2. Materials and methods: Protein sequence of nucleocapsid antigen from SARS-CoV-2 was expressed in E. coli BL21 and then was purified by chromatography. The purified protein was confirmed by SDS-PAGE and Western blotting. An indirect ELISA method was designed to measure the specific IgG and IgM antibodies against SARS-CoV-2 using recombinant N protein. The optimized ELISA method was then applied to measure the IgG and IgM antibodies in 61 infected or recovered COVID-19 patients and in 31 healthy controls. Finally, data obtained from the designed ELISA method were compared with those of a commercially approved ELISA kit. Results: The recombinant nucleocapsid protein was successfully expressed and purified which was confirmed by SDS-PAGE and Western blotting. The amount of optical densities obtained from the designed ELISA method was similar to those of the commercial kit in 61 patients and 31 controls. The sensitivity and specificity of the designed ELISA method for IgG were 100% compared with the commercial ELISA kit, while the sensitivity and specificity for IgM were 96.72 and 96.77, respectively. Conclusion: Serological tests alone are not suitable for diagnosis; however, their combination with molecular tests increases the accuracy and sensitivity of the COVID-19 diagnosis. These tests are also vauable for epidemiological studies.