India has begun to use the technology of antigen testing for Covid-19. This is faster and cheaper, than the “gold standard” of testing, the RT-PCR technology. It also very accurately detects positive cases of Covid-19. But its downside is that it gives a substantial number of false negatives where a person’s test can come as negative, when they could actually be positive for the viral disease.
“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,” said Satyajit Rath, former scientist at the National Institute of Immunology, New Delhi. “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.”
In other words, while antigen testing picks up a large number of positive cases quickly, some positive cases could remain undetected due to the antigen tests not being sensitive enough.
“Our positivity percentage from a month ago may not then be strictly comparable to our positivity percentage today,” said 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.”
In fact, many states are not providing the break-up of how many of tests are via RT-PCR and how many via antigen testing (see bulletin from Maharashtra here and Tamil Nadu here), instead publishing aggregate testing figures. The central government’s testing figures also do not show this detail – as per the Indian Council of Medical Research, 18.83 million samples have been tested for Covid-19 in India, upto July 30.
Let’s take a closer look, with the example of Delhi. The Delhi government has been testing more people with the antigen test than the RT-PCR test. The Delhi government informed the Delhi High Court in July that 478,000 antigen tests were done between June 18 and July 29. Only 180,000 RT-PCR tests were done in the same period. 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 antigen testing?
Like the RT-PCR test, and unlike antibody tests, antigen testing looks for the presence of the SARS-CoV-2 virus in the body. In an antigen test, a swab is taken from a person’s nasal cavity and it is tested to detect fragments of proteins which are found on or within the SARS-CoV-2 virus. The RT-PCR test also looks for the presence of the virus, but here the samples are tested for genetic material of the virus. (The antibody test is different–in this, blood samples are tested for the presence of antibodies that develop while or after a person has developed some level of immune response to the virus).
RT-PCR tests are “incredibly accurate”. Antigen tests are not as sensitive, and the US drug regulator says this aspect of antigen testing means that “positive results from antigen tests are highly accurate, but there is a higher chance of false negatives, so negative results do not rule out infection,” and need a confirmation via the RT-PCR test.
In its defense, the antigen test gives results much faster. While the RT-PCR test may take two to five hours at the least, the antigen test can give a result in 30 minutes.
“Antigen testing is important for diagnosis because it gives a high level of accurate positive results, cheaply and fast,” said Vineeta Bal, visiting faculty at 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.”
What’s the government’s stand?
The Indian government has said that those with symptoms of Covid-19 can get tested with the antigen test. But if their result comes out negative, a re-test using the RT-PCR technology must be undertaken. This was explained in a circular on June 23, about “newer additional strategies for Covid-19 testing”.
The document explained that antigen testing can help at “field level for early detection of infection and quick containment” and can give a “quick diagnosis of SARS-CoV-2 in field settings”. The government then said that this technology should be used both in field settings (outside of the laboratory) as well as in hospitals, and along with the RT-PCR test technology.
So far, antigen tests manufactured by at least three companies have been approved by the ICMR for use in India.
So what’s the problem?
For all its advantages, the antigen test’s low sensitivity, or the higher likelihood of false negatives, is its big problem. For example, the first antigen test approved by the ICMR was made by SD Biosensor, a South Korean company manufacturing in Gurugram, India. This test kit showed a high specificity (ability to detect true negatives), but a relatively low sensitivity (ability to detect true positives). Its specificity was in the range of 99.3% to 100% and sensitivity in the range of 50.6% to 84%. This means that when it returns a positive result, it is 99.3% to 100% accurate, but when it returns a negative result, it could be only 50.6% to 84% accurate.
This can skew trends, for example when states such as Delhi begin relying more on antigen testing than RT-PCR testing to detect Covid-19, and start showing fewer cases.
So even though antigen testing is very useful to detect new positive cases of Covid-19, the fact that many of its results could be false negatives can directly impact how good a city looks on Covid-19 charts. Mixing up these two data points could give an impression that a state is testing widely and that its number of positive cases is falling or that the increase in case numbers is slowing, when in fact that may not be the case.
This article first appeared on IndiaSpend.
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