BioRad's new, broader antibody test promises to improve COVID-19 detection, treatment

A new blood assay, designed to detect four different antibodies associated with SARS-CoV-2 infection, promises to increase diagnostic accuracy and to help researchers understand the spread of COVID-19. 

Most existing COVID-19 tests focus on identifying whether the immune system has made antibodies against a single antigen, such as the spike protein, the nucleocapsid, or the receptor-binding domain.

The new test, designed by BioRad Laboratories of Hercules, California, is known as a multiplex assay. It tests for the presence of antibodies to four functionally distinct proteins in the viral particle. These are: 

The two sub-units, S1 and S2, of the exterior spikes which the virus uses to invade the host cells.

The nucleocapsid, the container that holds the viral material.

And the receptor binding domain, a part of the S1 subunit of the spike that recognizes and binds to the host cell.

Now a group of 10 researchers, eight from the University of Pittsburgh and two from BioRad Laboratories, has completed a study of the effectiveness of the BioRad multiplex assay. A report on their study of 752 blood samples was accepted by the journal Clinical Biochemistry and became available online Aug. 26. 

The BioRad assay was found “comparable to existing assays, and achieved 100% sensitivity when all markers were included,” according to the authors. Sensitivity is a measure of a test’s ability to identify positive cases. So 100% sensitivity means that all positive cases will be identified by the test, but it may also include some false positives. 

The assay also exhibited an extremely high specificity, a measure of the number of false positives, when applied to a group of 170 samples selected for probable interference by natural infection. 

The authors broke down the specificity for this group of samples by antibodies. Specificity for the spike unit 2 (S2) and the nucleocapsid antibody (N IgG) came to 100%, while specificity for the receptor binding domain (RBD) antibodies and the spike subunit 1 (S1 IgG) was 99.4%. The latter group contained one false positive, which was determined to be due to infection by one of the other common coronaviruses. 

One of the hoped-for advantages of the multiplex assay is that it can differentiate immunity acquired by vaccination from that induced by natural infection. The high specificities achieved are therefore very promising for the assay.

Overall, the researchers found the BioRad multiplex assay “to be acceptable for clinical use” and comparable to existing European- and U.S.-approved assays. 

Among the limitations of their study, the researchers noted the need for “additional freeze-thaw cycles” in the samples they assayed, a “relatively small cohort size,” and “a lack of demographic and comorbidity information.”

As to its advantages, they said, “The ability to measure multiple analytes simultaneously may be advantageous for complex clinical presentations, epidemiologic research to differentiate between natural disease and some vaccines, and in some decisions regarding infection prevention strategies.”

The authors conclude by calling for “additional independent validations . . . to further determine RBD binding antibody and neutralizing antibody correlations and corroborate these findings.”

Nathan Cook, et al. Multiplex assessment of SARS-CoV-2 antibodies improves assay sensitivity and correlation with neutralizing antibodies. Clinical Biochemistry (2021). DOI: https://doi.org/10.1016/j.clinbiochem.2021.08.006