Scientists have developed a new accurate and reliable test to detect antibodies for SARS-CoV-2, the coronavirus that causes Covid-19, which gives results in less than 20 minutes.
The serological assay, described in the journal Scientific Reports is as accurate as the most reliable antibody tests currently available, but is less complex and can be performed much faster.
The gold standard for serological testing is a complex laboratory method called ELISA that takes four to six hours to run and provides quantitative results indicating the strength of the immune response.
Simpler assays using test strips provide rapid results, but are less reliable and do not quantify antibody levels, the researchers said.
The new method, called biolayer interferometry immunosorbent assay (BLI-ISA), provides complete quantitative results in less than 20 minutes.
"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, an associate professor at the University of California, Santa Cruz, US.
"It combines the advantages of the test strips that take 20 minutes with the quantitative results and higher performance of ELISA," DuBois said.
A positive antibody test indicates prior infection with the virus. These tests are not used to diagnose active infections, however, which requires a different test that detects the virus's genetic material or the virus antigens.
Antibodies are proteins made by the immune system that recognise and bind to virus antigens.
Serological testing is important for understanding the spread of the coronavirus by determining how many people in a population have been infected.
The tests are also used to evaluate the responses to experimental vaccines in both people and laboratory animals.
Researchers said quantitative information about antibody levels could be especially important in the future if scientists are able to determine that a certain level of particular antibodies is needed to provide protection against infection with the coronavirus.
"That is still to be determined, but we do know that people who have been infected with SARS-CoV-2 have very diverse levels of antibodies, and it would not be surprising to find that below some baseline level they might not be protective," DuBois said.
"So it's really useful to have that quantitative ability to know what someone's antibody status is, whether it's from a past infection or a vaccination," she said.
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 researchers said.
The new serological assay involves several steps performed by the instrument in an automated "dip-and-read" format, they said.
In the first step, the biosensor tip is dipped into a solution containing the antigen -- a viral protein -- that is recognised 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.
After dipping into a wash solution, the biosensor is dipped into the blood plasma sample, generating a signal as antibodies bind to the antigen.