Quantitative Measurement of IgG to SARS-CoV-2 Proteins Using the Phadia ImmunoCAP 250

Keshavarz, Behnam; Wiencek, Joseph; Bazydlo, Lindsay; Straesser, Matthew; Workman, Lisa; Platts-Mills, Thomas; Wilson, Jeffrey

Rationale: Measurement of IgG to SARS-CoV-2, the cause of coronavirus disease 2019 (COVID-19), has an important role to play in understanding the epidemiology and immune response to the novel coronavirus. To date, most commercial antibody assays have not had a quantitative read-out. Methods: SARS-CoV-2 spike-receptor-binding-domain (RBD) and nucleocapsid proteins were biotinylated and used on the solid-phase of the ImmunoCAP using the biotin-streptavidin technique. The assay was developed using plasma from 15 patients hospitalized with COVID-19 and serum from 86 subjects recruited prior to the emergence of SARS-CoV-2. Assay quantitation was validated using a commercially available monoclonal anti-SARS-CoV-2 Spike-RBD antibody (CR3022). Specific IgG was subsequently measured in 17 patients hospitalized with COVID-19 in which serum was available at day 0 and 7 post-admission. Results: The cut-off limit for the assays (mean±2SD) was determined using pre-COVID-19 samples as 4.9 μg/mL for spike-RBD and 4.6 μg/mL for nucleocapsid. Based on these cut-offs, the assay performance characteristics were: 100% sensitivity and 97.7% specificity for spike-RBD and 100% sensitivity and 97.6% specificity for nucleocapsid. Experiments using known concentrations of CR3022 correlated strongly with the assay results (R2=0.998). Among the 17 patients there was a wide variation in the magnitude of IgG responses. At day 7 of admission median values were 86.4 μg/mL [range 3.2-158.0] and 38.6 μg/mL [range 0.6-191.0] for spike-RBD and nucleocapsid, respectively. Conclusions: We have described a quantitative assay to measure IgG to SARS-CoV-2. The assay has a read-out in standardized units (μg/mL) and could be adopted for use in clinical laboratories.