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Hong Kong-Based Team Develops Rapid Microfluidic SARS-CoV-2 Antibody Test


NEW YORK – A team of researchers from Hong Kong has developed a test that can rapidly determine a person's SARS-CoV-2 antibody status, indicating their immunity to the virus.

According to Ting-hsuan Chen, an associate professor in the department of biomedical engineering at City University of Hong Kong who helped develop the assay, the level of antibodies in someone's blood implies the level of protection they have against the virus after either vaccination or infection. Measuring those antibodies can help people understand whether they'll need an additional vaccine booster, since antibody levels "decay" over time, but there's "not a very convenient tool" to quantitatively measure antibodies in a sample, he said.

While ELISA immunoassays are the gold standard for sensitivity and quantification, they are expensive and time-consuming and usually require a central laboratory — while there are some portable ELISA systems, they can still be pricey.

Another option to measure antibodies is lateral flow testing, which is significantly cheaper and easier to manufacture and transport but doesn't provide a quantitative result.

"To visualize or quantify, that's the question," Chen said. ELISA is the better choice for accuracy, while lateral flow wins when it comes to convenience, he added.

Knowing the limitations of both kinds of testing, Chen and his team set out to create a test that would be comparable to an antigen test in cost and portability but have the sensitivity and quantification of an ELISA assay. In a paper published on Friday in Science Advances, Chen and his co-authors laid out their method for developing a microfluidic-based SARS-CoV-2 antibody test that delivers results within 20 minutes.

The test relies on two kinds of particles — magnetic microparticles and polystyrene microparticles — that bind to SARS-CoV-2 antibodies when they're present in a sample. To run the test, a finger-blot sample of serum or plasma, equivalent to about 5 microliters, is mixed with a solution that contains both types of particles for incubation, then loaded onto a microfluidic device. The solution, now containing the sample and microparticles, flows through a magnetic separator that removes the magnetic particles and bonded polystyrene microparticles, leaving only free polystyrene microparticles that haven't bonded with magnetic microparticles. Those polystyrene microparticles flow into a dam, and the more polystyrene microparticles present in that dam, the lower the antibody level. The number of free particles present is indicated by a single line of particle accumulation that can be seen with the naked eye, which can then be compared to a chart that correlates the length of the line to the concentration of antibodies.

There are two options for that final particle dam, Chen noted: a "rapid mode" and a "sensitive mode." The rapid mode returns results in 20 minutes, while the sensitive mode takes about 70 minutes. The longer incubation time allows more antibodies to bind to microparticles, ensuring the result is more accurate, particularly in samples where the concentration level is low.

In the study used for the Science Advances paper, Chen's team looked at samples from 91 volunteers who had received either of the two main kinds of vaccines used in Hong Kong: Pfizer and BioNTech's codeveloped vaccine, and Sinovac Biotech's CoronaVac. The researchers originally used an ELISA instrument to measure the antibody concentration sample that they then compared to their test and found "a similar result in the measured antibody levels," Chen said. The team also brought back 10 volunteers after 45 days to remeasure the antibody levels to see if the concentrations had dropped — for the BioNTech patients, the antibody levels dropped but there was still some protection, while the CoronaVac patients had very few antibodies left.

While this study was only conducted on people with antibodies from vaccination, Chen said that the researchers would like to validate it with samples from people who were previously infected to see if it works the same way. However, because Hong Kong had had low infection rates leading up to the time the study was conducted, finding enough samples proved difficult — the recent Omicron outbreak may change that, though, and provide the opportunity to gather more data on naturally infected people.

The major use Chen sees for the test is for people to know their "level of protection" post-vaccine, and he said he thinks it could also be a "good index" to determine when another booster may be necessary. On the policy side, it could be used similarly to the vaccine passports that are required by some countries to travel abroad and return home — a sort of "immunity passport."

"We know a vaccination record doesn't mean much for the level of protection" due to the speed with which SARS-CoV-2 antibodies leave our bodies, so this test could allow for a "clear separation of high risk and low risk," he said.

In its current format, the test would most likely be used at local clinics or similar point-of-care settings, although it could conceivably be used at home if the manufacturing were cheap enough. Thus far, all of the development work has been done in an academic lab, but Chen said he is actively seeking a manufacturing partner and hoping the publication could bring some awareness. That manufacturing partner could help the team change the material it's using for the test — that material currently isn't compatible with the needs for mass production.

The device may also need another method of filtration, due to the difficulties of working with whole blood. To get to the plasma containing antibodies from a blood sample, the red blood cells need to be filtered out — a tricky proposition, and one the team is currently working on by building a device with an integrated filtration method. To bypass that problem, the researchers are also working to utilize the test with other sample types that contain antibodies, such as saliva and nasal secretions. "The device works better for other specimen types because there are no red blood cells, so no filtration" is required, he said.

Another obstacle to adoption of the test developed by Chen and his colleagues is questions about the usefulness of antibody-based tests for SARS-CoV-2. To date, such assays have not been widely adopted as detecting a past infection has not been viewed as clinically urgent. In addition, not enough is known about how exactly antibody levels correlate with immunity.

Aside from using antibodies, some firms are also developing T-cell-based tests to measure individuals' immune status associated with SARS-CoV-2, including Adaptive Biotechnologies. Such tests are believed to offer a more comprehensive view of immune status, but according to Chen, "for tests involving live cells, the professional laboratory including live cell incubator or biosafety cabinet is needed. In contrast, our detection of antibody level does not need to handle live cells, so it has higher potential for home [testing] of immunity."

SARS-CoV-2 is also "only the start of the device," Chen said. The test and technology aren't limited to COVID-19 but could be used for other infectious diseases by changing the binding particles and adapting them to another type of antibody. "We could make it for any other outbreak."

Since the key tenet of the device involves detecting the presence of a protein, it could also be used for cancer or other disease detection, which the team is investigating now. They're specifically looking at its potential usage with liver cancer biomarkers, prostate specific antigens, and Alzheimer's disease biomarkers, he said.