SCIENCE BEHIND ANTIBODY TESTS

The test widely used to detect coronavirus disease (Covid-19) is the realtime-polymerase chain reaction (RT-PCR), which detects genetic material of Sars-CoV-2 in oral or nasal swabs. It’s highly accurate, but only returns a positive result if the person is infected at the time of being tested. For people to have a detectable viral load, testing is recommended for people with symptoms or contacts of known cases, who should get tested between day five and day 14 of coming in contact with a confirmed case.The average detection time for RT-PCR five to six hours, which is the time taken to run one set of tests. It can also identify infections in people with no symptoms.

BLOOD TESTS

■ Since February, dozens of prospective serological (blood) test kits for Covid-19 have been developed around the world.

■ Most of these are enzyme-linked immunosorbent assay (ELISA), a commonly used blood test for detecting existing infections, like dengue and HIV. These are blood tests that work by mixing a blood sample with a highly-sensitive solution containing proteins from the virus.

■ If antibodies to Covid-19 are present in the person’s blood, they will bind to the proteins, triggering a colour change.

■ Antibody tests indicate whether a person has ever been infected, including in the past when they may not have known they were infected because they had mild or no symptoms.

■ But it takes eight and 11 days after infection for people to develop antibodies to the virus.

PROS AND CONS

■ Antibody testing cannot replace the more accurate RT-PCR, which just takes a couple of hours longer to produce results.

■ Blood test does not require handling of infectious virus and can be done in like a simple blood test.

■ Blood tests testing for antibodies can help determine how many people have been infected in a population, define previous exposure, and identify highly reactive human donors to develop convalescent serum to treat patients, and to evaluate the effectiveness of the potential vaccines. Countries like Singapore are using blood tests as part of the national surveillance programme of Covid-19 to determine how many people have been infected at the population level.

■ Mass testing health workers can identify those who are already immune, so they can be deployed to care for infected patients to minimise the risk of Covid-19 outbreaks in hospitals.