Science Spotlight

Validating antibody tests to determine SARS-CoV-2 seroprevalence

From the University of Washington and the Jerome lab, Vaccine and Infectious Disease Division

As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus behind coronavirus disease-19 (COVID19), arrived in the U.S. in early 2020, the roll out of reverse-transcriptase PCR (RT-PCR) diagnostic tests lagged severely behind transmission. As a result, many SARS-CoV-2 infections since went initially undiagnosed, and a past infection cannot be diagnosed through RT-PCR. Although virus is cleared from an infected person’s body within weeks, anti-SARS-CoV-2 antibodies remain in the blood for months, serving as a marker for those who have been previously infected. The ability to detect these antibodies is now crucial for determining the seroprevalence of SARS-CoV-2 in the population and to inform COVID-19 epidemiology and vaccine design.

While several serological assays for SARS-CoV-2 are have thus far been approved for diagnostic use, the existing tests have inadequate sensitivity and specificity to perform widespread clinical testing for a SARS-CoV-2, which has low seroprevalence in the greater population. Recently, Abbott designed a SARS-CoV-2 test that detects IgG against the virus nucleocapsid protein. Researchers from the Department of Medicine and Greninger lab at the University of Washington, along with Jerome lab colleagues from the Vaccine and Infectious Disease Division, validated the Abbott SARS-Cov-2 IgG test on the Abbott high throughput Architect instrument. They then used the assay to measure the SARS-CoV-2 IgG seroprevalence among a community in Boise, Idaho. These results were recently published in the Journal of Clinical Medicine.

Led by Dr. Andrew Bryan, this study first determined the performance characteristics of the Abbott serological test by testing serum from patients negative for SARS-CoV-2 as well as serum from patients with previous RT-PCR-confirmed SARS-CoV-2 infection. Of 1,200 SARS-CoV-2-negative samples, only one sample tested falsely positive based on manufacturer IgG index cutoffs, demonstrating an assay specificity of 99.9%. Using serum from confirmed SARS-CoV-2-positive patients, the authors tested sensitivity at days 7, 10, 14, and 17 after both onset of symptoms and RT-PCR diagnosis. Although sensitivity was higher in the samples measured post-diagnosis (53.1% at day 7 post-symptom onset and 88.7% at day 7 post-diagnosis), the Abbott test achieved 100% sensitivity by day 17 in both groups.

After confirming the validity of the Abbott Architect SARS-CoV-2 serology assay, Bryan and colleagues assessed the SARS-CoV-2 seroprevalence in a cohort of 4,856 self-selected people sampled in April 2020 in Boise, Idaho as part of the Crush the Curve campaign. They found the cohort seroprevalence to be 1.79%, with the male faction holding slightly higher seroprevalence (2.1%) than the female (1.6%). Seroprevalence increased with age, peaking at 4% among those over 80 years old and as low as .4% in individuals under the age of 19.

A. Serum IgG measurements are reproducible and consistent across biologic replicates, except in samples with low IgG index values or during seroconversion.  B. IgG index values at increasing days since COVID-19 symptom onset in seroconverting individuals.
A. Serum IgG measurements are reproducible and consistent across biologic replicates, except in samples with low IgG index values or during seroconversion. B. IgG index values at increasing days since COVID-19 symptom onset in seroconverting individuals. Figure provided by Dr. Keith Jerome.

This work validated that the Abbott Architect performance characteristics match manufacturer claims and can detect SARS-CoV-2 IgG with high specificity and sensitivity, proving that this test can be used to accurately detect recent infection with SARS-CoV-2. This screening will be necessary to identify those who may have serological immunity to the virus, inform vaccine design, and to understand the early 2020 transmission dynamics of the pandemic. The validation of the Abbott Architect serological assay provides a substantial improvement in the collective ability to detect the infections that were missed during active infection. However, it is still unknown if individuals with asymptomatic infection retain appreciable antibody and if the Abbott Architect assay can detect it. Furthermore, additional serological tests will be needed to detect recombinant vaccine-elicited antibodies against other viral antigens such as spike proteins.

This work was supported by supported by the Department of Laboratory Medicine at the University of Washington Medical Center.

UW/Fred Hutch Cancer Consortium member Keith Jerome contributed to this work.

Bryan A, Pepper G, Wener MH, Fink SL, Morishima C, Chaudhary A, Jerome KR, Mathias PC, Greninger AL. 2020. Performance Characteristics of the Abbott Architect SARS-CoV-2 IgG Assay and Seroprevalence in Boise, Idaho. Journal of Clinical Medicine. DOI: 10.1128/JCM.00941-20