SARS-CoV-2 antibody characterization in emergency department, hospitalized and convalescent patients by two semi-quantitative immunoassays

Abstract

Background: In the ongoing COVID-19 pandemic, there is an urgent need for comprehensive performance evaluation and clinical utility assessment of serological assays to understand the immune response to SARS-CoV-2.

Methods: IgM/IgG and total antibodies against SARS-CoV-2 were measured by a cyclic enhanced fluorescence assay (CEFA) and a microsphere immunoassay (MIA), respectively. Independent performance evaluation included imprecision, reproducibility, specificity and cross-reactivity (CEFA n = 320, MIA n = 364). Clinical utility was evaluated by both methods in 87 patients at initial emergency department visit, 28 during subsequent hospitalizations (106 serial samples), and 145 convalescent patients. Totally 916 patients and 994 samples were evaluated.

Results: Agreement of CEFA and MIA was 90.4%-94.5% (Kappa: 0.81-0.89) in 302 samples. CEFA and MIA detected SARS-CoV-2 antibodies in 26.2% and 26.3%, respectively, of ED patients. Detection rates increased over time reaching 100% after 21 days post-symptom onset. Longitudinal antibody kinetic changes by CEFA and MIA measurements correlated well and exhibited three types of seroconversion. Convalescent sera showed a wide range of antibody levels.

Conclusion: Rigorously validated CEFA and MIA assays are reliable for detecting antibodies to SARS-CoV-2 and show promising clinical utility when evaluating immune response in hospitalized and convalescent patients, but are not useful for early screening at patient's initial ED visit.

Keywords: Coronavirus disease 19 (COVID-19); Immunoassay; Serology; Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Fig. 1

Chart of numbers of patients and samples used in the study.

Fig. 2

Antibody index values (log10 scale) of samples from 42 SARS2-CoV RT-PCR positive patients measured by CEFA and MIA methods over time. Each antibody level measured by CEFA (a) and total antibody level measured by MIA (b) was plotted over days after any symptom onset. Longitudinal samples from each individual patient measured by CEFA were connected by the solid (IgG, solid cirles) and dotted (IgM, solid triangles) lines measured by CEFA (c). Longitudinal samples from each individual patient measured by MIA were connected by the solid (total antibodies) line (d). Average line predicted by GEE method.

Fig. 3

Antibody kinetic changes of SARS2-CoV RT-PCR positive patients who were on ventilators (a) and not on ventilators (b). Dashed black line is the normalized index values of positive cutoff. —□—CEFA IgG; —ο— CEFA IgM; —Δ—MIA total antibodies.

Fig. 3

Antibody kinetic changes of SARS2-CoV RT-PCR positive patients who were on ventilators (a) and not on ventilators (b). Dashed black line is the normalized index values of positive cutoff. —□—CEFA IgG; —ο— CEFA IgM; —Δ—MIA total antibodies.

Fig. 4

Distribution of positive antibody levels in convalescent serum measured by CEFA for IgG (log10 scale, 4a) and MIA for total antibody (4b).