In the race for better equipment and technologies to fight the COVID-19 pandemic, India has begun using the antigen testing for COVID-19, which is claimed to be faster and cheaper than the RT-PCR technology. It accurately detects a substantial number of positive cases. But on the contrary, even if a person with infected with the virus, a lot of false-negative results can come.
“The problem is we are saying there is a certain percentage of positivity in testing and as positivity in testing goes down, we may feel that we have begun to flatten the curve,” says Satyajit Rath, former scientist at the National Institute of Immunology in a report to IndiaSpend. “Have we really? Because if we have shifted to what is possibly a less sensitive test, like antigen testing, then the reduction in the percentage of test positives can simply be because of this.”
“Our positivity percentage from a month ago may not then be strictly comparable to our positivity percentage today,” says Rath. “So, if we shift from one kind of testing to another and there is no break-up given of how much is by antigen testing, we no longer have comparable data.”
On similar lines, many states have not provided separate data for how many of the tests are via RT-PCR and how many via antigen testing. According to the Indian Council of Medical Research (ICMR), 18.83 million samples have been tested for COVID-19 in India, till July 30.
Between June 18 and July 29, a total of 478000 antigen tests were done in Delhi, reported the Delhi Government to the Delhi High Court. Of the 478,000 people tested with antigen tests, 2,818 were re-tested with RT-PCR tests and 404 of them showed up as positive.
What is the antigen testing?
The antigen testing looks for the presence of the SARS-CoV-2 virus and is tested to detect the fragments of proteins that are found on and within the virus. The positive results from the antigen tests are highly accurate but there is a higher chance of false negatives as well. In its defense, the antigen tests are much faster and can give results within 30 minutes of testing.
“Antigen testing is important for diagnosis because it gives a high level of accurate positive results, cheaply and fast,” says Vineeta Bal, visiting faculty at the Indian Institute of Science Education and Research, Pune. “But high testing with this test by itself is not good enough if those who test positive are not also isolated and treated so that the transmission chain can be broken.”
The Indian take on antigen tests
The Indian Government has devised a strategy for using antigen testing. Those with the symptoms for COVID-19 can get tested with the antigen test, but people with negative results must undertake RT-PCR testing. This was explained in a circular on June 23, 2020, about “newer additional strategies for COVID-19 testing”.
The circular also stated that “field level for early detection of infection and quick containment” and can give a “quick diagnosis of SARS-CoV-2 in field settings”. So far, antigen tests manufactured by at least three companies have been approved by the ICMR for use in India.
The problems with the antigen testing
Among multiple problems with the antigen testing, the biggest problem is the test’s low sensitivity or a high chance of false negatives. The first testing kit approved by ICMR, which was made by a South Korean company named SD Biosensor, showed high specificity but low sensitivity. The specificity was in the range of 99.3% to 100% and sensitivity in the range of 50.6% to 84%, meaning that the chances of positive results being accurate is 99.35 to 100% and the chances of negative results being accurate ranges between 50.6% and 84%.
The testing is also likely to affect the data for the whole crisis with states using antigen testing showing fewer results if applied on a large scale. It directly impacts how good a city looks on COVID-19 charts.