Selection of a SARS-CoV-2 antibody quantification method and development of an antibody reference standard for ELISA to test immunoglobulin preparations

Abstract

The development of COVID-globulin, a COVID-19-specific human immunoglobulin preparation, involved choosing a method to quantify antibodies to SARS-CoV-2. Antibody titre determination by virus neutralisation (VN) is labour-intensive and unsuitable for large-scale application. To enable routine testing, it was necessary to develop a less demanding method; the enzyme-linked immunosorbent assay (ELISA) was the most appropriate of solutions. The lack of international and industry reference standards (RS) prompted the preparation and certification of an RS for COVID-globulin potency control.The aim of the study was to examine the possibility of substituting ELISA for VN and to develop an RS for SARS-CoV-2 antibody quantification in immunoglobulin preparations.Materials and methods: the authors used commercial ELISA kits by several manufacturers, COVID-globulin by Microgen (48 batches), and human plasma samples from multiple sources (1499 samples). The tests were performed by VN, ELISA, and chemiluminescent microparticle immunoassay.Results: the authors validated an ELISA method for SARS-CoV-2 antibody quantification with the selected reagent kits by the National Medical Research Center for Hematology (NMRC for Hematology) and Euroimmun AG. The authors demonstrated the possibility of using ELISA instead of VN (with a correlation coefficient of more than 0.9). They developed and characterised an in-house RS for SARS-CoV-2 antibody content in human immunoglobulin preparations. The RS was certified in newly introduced anti-COVID units (ACU) and in international binding antibody units (BAU) using the World Health Organisation (WHO) international reference panel (NIBSC code: 20/268). The RS's potency was measured in terms of its neutralising activity in ACU (320 ACU/mL) and BAU (2234.8 BAU/mL). The authors established the relationship between ACU and BAU units. For the selected ELISA reagent kits, the conversion factors were 6.4 (NMRC for Hematology) and 7.0 (Euroimmun AG).Conclusions: the ELISA method for SARS-CoV-2 antibody quantification and the RS for SARS-CoV-2 antibody content can be applied to determine the potency of human anti-COVID-19 immunoglobulins.