New Serological Assay Uses Optical Biosensor to Measure Antibody Levels in Blood Samples within 20 Minutes

The new BLI-ISA method uses biolayer interferometry, an optical technique for measuring the interactions between molecules by detecting the binding of molecules to the tip of a fiber-optic biosensor. The instruments required to perform biolayer interferometry are increasingly common in research laboratories.

The new serological assay involves several steps performed by the instrument in an automated “dip-and-read” format. In the first step, the biosensor tip is dipped into a solution containing the antigen (a viral protein) that is recognized by the antibody to be tested for. As the antigen binds to the biosensor tip, it generates a signal that can be used for quality control to ensure consistency in the antigen loading step. Next, after dipping into a wash solution, the biosensor is dipped into the blood plasma sample, generating a signal as antibodies bind to the antigen.

The immune system makes different types of antibodies, called isotypes, including IgM, which is produced early in the infection and declines later, and IgG, which is produced later and persists longer. In the antibody binding step, the BLI instrument detects any type of antibody that binds to the antigen. The next step detects and quantifies IgG antibodies specifically by measuring the binding of anti-IgG antibodies. The assay thereby provides quantitative measurements of both total antibodies and IgG antibodies, and it can be designed to measure different isotypes as well.

The researchers developed and tested the new assay using RBD antigens, although it can be used to detect a wide range of antigens. The BLI instrument allows easy “multiplexing,” meaning that multiple tests can be run in parallel on the same blood sample to detect antibodies to different viral antigens as well as different antibody isotypes. Because it requires laboratory equipment, BLI-ISA could not be used as a point-of-care test at doctor’s offices or pharmacies. However, it allows high-throughput processing of samples and is faster and less labor intensive than other quantitative laboratory tests such as ELISA, Immunofluorescent Assay, and Chemiluminescent Immunoassay. The amount of blood needed for the test can be obtained with a finger prick.

Another advantage of BLI-ISA over other methods is the ease of standardizing the assay so that a sample gives the same quantitative results in repeated tests at different times or in different laboratories. ELISA requires an enzyme-based signal amplification step, which can vary depending on temperature and other factors, resulting in much more test-to-test variability in the quantitative measurements. The researchers believe that the new assay could be especially useful for researchers in vaccine development.

“Our assay is as sensitive if not better than other assays in detecting low levels of antibodies, and the specificity [false-positive rate] is as good as the best antibody tests out there,” said Rebecca DuBois, associate professor of biomolecular engineering at UC Santa Cruz. “It combines the advantages of the test strips that take 20 minutes with the quantitative results and higher performance of ELISA.”

“This method provides a standardized way to quantify antibody levels, which could be used to compare antibody responses to different vaccine candidates,” said first author John Dzimianski, a postdoctoral researcher in DuBois’s lab. “In addition, running the assay itself is straightforward, requiring little more than the push of a button. It’s a simple but powerful tool.”

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