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Antigen, PCR and antibody: All you need to know about the various COVID-19 tests

While they have similar names and provide results rapidly between 15-30 minutes, the purpose of antigen and antibody testing is completely different

Published: 20th July 2020 08:43 PM  |   Last Updated: 20th July 2020 08:43 PM   |  A+A-

Rapid antibody testing kit

Rapid antibody testing kit

Express News Service

CHENNAI: Recently, the Chennai Corporation began COVID-19 antibody testing for its staff while the Tamil Nadu government too has announced that it will be conducting sero-surveillance -- which aims to detect antibody levels against the virus to measure the population's immunity.

On the other hand, the Indian Council of Medical Research had given the go-ahead for all states to use the rapid antigen based testing to detect the virus. However, due to concerns over its efficacy, the state decided not to procure the kits but instead use the gold standard RT-PCR kits.

While antigen and antibody testing have been in the news lately, they have created confusion among netizens, with many under the impression that both are the same. While they have similar names and provide results rapidly between 15-30 minutes, the purpose of each is completely different.

Here, we explain the difference between the various testing methods -- PCR, antigen and antibody -- and also explore the possibilities of newer antibody tests.

Antigen vs PCR testing

Virologist Dr Jacob John says the antigen test picks up the outside coat protein of the virus when it is present in the body. "While a PCR test picks up the gene inside the virus, the antigen test picks up the part outside the virus. Both the tests are done by taking nasal swabs," says Dr John.

Dr John says that both the PCR and antigen tests can pick up dead virus particles too.

"However, PCR remains the gold standard as it can pick up virus presence when the viral load is low whereas the antigen needs at least 1000 viral particles for it to show positive," he adds.

A PCR test takes at least up to six hours to produce results, while an antigen test can produce results in 15-30 minutes.

As per the ICMR's notification, antigen kits have a low sensitivity ranging from 50.6 percent to 84 percent. Because of this, the ICMR has said people who are negative must be re-tested with RT-PCR, while those who are positive need not undergo it. This is another reason why Tamil Nadu will not be using the kits.

Antibody testing and its challenges

However, antibody tests, explains Dr John, are done mostly by taking blood samples of antibodies which are generated after an infection. "They detect only the past infection by identifying the antibodies Immunoglobulin M, which comes a week after the infection, and Immunoglobulin G which comes after IgM," he says.

Dr John says antibody tests are carried out mainly for sero-surveillance to identify the spread in the community.

Sero-surveillance is done by antibody testing and if a person has the IgG antibody, it indicates that he or she has recovered. This helps in identifying the spread and immunity and also in selecting candidates for plasma donation.

There are some challenges with the antibody kits as well, say experts.

"A commercial antibody kit only detects IgM and IgG antibodies produced by the B-cells but it does not detect antibodies produced by T lymphocytes Cells. The latter is long-lasting," says Dr Subramanian Swaminathan, Infectious Diseases Specialist at Gleneagles Global Health City. T cells, produced in the thymus gland, are part of the human immune response against viral infections. The cells directly target and kill infected cells.

A recent Singapore study published in Nature has found that subjects who recovered from COVID-19 had SARS-CoV-2-specific T cells.

The study also examined uninfected individuals and found the presence of SARS-CoV-2 specific T cells in more than 50 per cent of them, concluding that it could be due to cross immunity from other coronaviruses.

Dr Swaminathan said that as commercial kits do not detect T-cell antibodies, if a person tests negative, it does not mean he is not immune to the virus.

"Those with a mild infection may test negative but they could still be immune as they may have T-cell antibodies," adds Dr Swaminathan.

He says that if the state is only going to use antibody tests to determine immunity, it is going to massively underestimate the level of immunity. "We need to develop kits for T-cell tests and have a look at the results," said Dr Swaminathan.

Meanwhile, the ICMR has capped the price of antigen kits at Rs 450 while an RT-PCR test in TN costs up to Rs 3,000. Antibody tests are being offered by private labs at prices beginning from Rs 600. However, the Corporation did not reveal the prices of the antibody kits it is using to test its staff.

Corporation Commissioner G Prakash said the antibody kits are procured from CSR funds and not from the civic body's budget. "The prices may vary," he said.

The state expects to carry out around 25,000 tests as part of sero-surveillance and the Chennai Corporation is set to carry out another 12,000 tests in its limits for this purpose.



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