New Delhi | Jagran News Desk: The novel coronavirus has warned us in more than just one way since the past months. As a result, we have been introduced and exposed to many new terminologies related to testing methodologies. Since the outbreak, different methods have been used by different states as per the ICMR guidelines to detect the virus.

Recently, 15% of symptomatic patients who tested negative for COVID-19 in the rapid antigen tests conducted in Delhi were found to be positive in a second confirmatory RT-PCR test, the Health ministry informed on Tuesday.

Although the Indian Council of Medical Research had given the go-ahead for all states to use the rapid antigen-based testing for detecting the virus, Tamil Nadu, which is the second-most affected state in the country after Maharashtra, decided not to procure the kits but instead use the gold standard RT-PCR kits due to the concerns regarding the efficacy of the rapid antigen kits.

As the talk of testing methods and kits goes on, it leads to confusion in many netizens, often giving the impression that both are the same. While antigen and antibody testing have similar names and give results between 15-30 minutes, the purpose of both of them is different.

How do antigen and antibody tests differ?

Antibody tests are carried out mainly for sero-surveillance to identify the spread of the virus in the community. The tests reveal if a person has already been exposed to an infection, by detecting antibodies in their blood or serum. Experts say that antibody tests 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. According to the data, the IgM antibody response peaks around two weeks after infection, followed by the IgG antibody peak at three weeks in the case of SARS-CoV-2.

On the other hand, the antigen test reveals if a person is currently infected with a pathogen such as the SARS-CoV-2 virus. Once the infection has gone from the body of an individual, the antigen disappears. Antigen tests detect proteins or glycans, such as the spike proteins found on the surface of the SARS-CoV-2 virus.
However, both of them provide results rapidly and are relatively cheap and can be more amenable to point-of-care use, which could make them more suitable for testing in the community and in remote regions. But antigen test can take longer to develop than molecular and antibody tests and accuracy also remains to be a problem in the case of antigen tests.

What is PCR testing and how is it different?

According to the ICMR's notification, antigen kits have a low sensitivity ranging from 50.6 percent to 84 percent, and due to this, the ICMR has laid out that people who test negative must be re-tested with RT-PCR, while those who are positive need not undergo it.

"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," Virologist Dr. Jacob John was quoted as saying by the New Indian Express.
Both the PCR and the antigen test take up the dead virus particles but 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, reports the New Indian Express.

While each one of these testing methods differ in many aspects and PCR remains to be the gold standard and sero-surveillance is carried out by an antibody test, There are some challenges with the antibody kits.
As compared with many other test methods, antibody test results are unfortunately not always correct.

Posted By: Simran Babbar