Cross-Sectional; Retrospective Cohort; Other
Study population and setting
This study evaluated the test performance of the Abbott SARS-CoV-2 IgG assay, which is a chemiluminescent microparticle immunoassay for the qualitative detection of IgG antibody against the SARS-CoV-2 nucleoprotein in plasma or serum. To assess assay sensitivity, 689 excess serum samples from 125 patients in Seattle who were PCR-positive for SARS-CoV-2 infection were tested. To assess assay specificity, 1,020 de-identified serum samples from unique individuals that had excess serum originally obtained for herpes simplex serology in 2018-2019 (i.e., pre-COVID-19) were tested. In addition, the study evaluated the seroprevalence of anti-SARS-CoV-2 among 4,856 individuals who were sampled in April 2020 via the Crush the Curve program in Boise, Idaho.
Summary of Main Findings
Sensitivity of the Abbott SARS-CoV-2 IgG assay increased with time since symptom onset and from the date of PCR positivity among mostly hospitalized COVID-19 patients. For this group, the assay had 100% sensitivity at 17 days post symptom onset and 14 days from the date of PCR positivity. In pre-COVID-19 serum samples (n=1,020), assay specificity was 99.0%; only one false-positive result was observed. Specificity of the assay could be improved to 100% while maintaining sensitivity at 100% by increasing the manufacturer’s cut-off value for seropositivity. Assay results were also shown to be reproducible. Among the participants in Boise, Idaho, anti-SARS-CoV-2 seroprevalence was 1.8% overall, 2.1% in males, and 1.6% in females.
The study used longitudinal sera to describe increases in antibody detection over time and assess the impact of infection duration on assay sensitivity. The sample sizes used to assess both assay sensitivity and specificity were large.
The specimens used to evaluate test performance characteristics of the Abbott SARS-CoV-2 IgG assay were not well characterized. Most of the sera used to assess assay sensitivity were from mostly hospitalized patients who were recently infected with SARS-CoV-2. It is unclear whether the Abbott SARS-CoV-2 IgG assay would have optimal sensitivity in mild or asymptomatic cases of SARS-CoV-2 infection or recovered COVID-19 patients who are no longer PCR-positive. Additionally, the study did not include specimens from patients known to be infected with other coronaviruses, so it is unclear whether the Abbott SARS-CoV-2 IgG assay produces false-positive results due to cross-reactivity. Finally, the Crush the Curve participants were a self-selected sample and are not representative of all Boise, Idaho residents, so the seroprevalence estimates may not be reflective of the general Boise population.
This study provides key test performance characteristics for a commercially-available SARS-CoV-2 IgG antibody assay and demonstrates its utility in community serosurveys.