Covid-19:Is RAT testing hiding Delhi's true pandemic story?

After rapid antigen tests (RATs) adoption, Delhi's positivity rate has come down. Earlier WHO had warned that RAT might not detect more than half the positive cases

Something unusual seems to have happened with Delhi’s Covid-19 testing strategy and infection statistics since Home Minister Amit Shah’s meeting with Chief Minister Arvind Kejriwal and Lieutenant General Anil Baijal on June 14. The national capital’s tests jumped 27 per cent that day, crossing 7,000 tests a day for the first time. By the time Shah and Kejriwal met again on June 18, Delhi which is home to 19 million people ramped up daily testing to over 13000 tests a day.

The Delhi government had become the first in India to permit the use of rapid antigen tests (RATs) for coronavirus in so-called containment zones. The kits used for sample collection — each comprising a Covid-19 antigen test device, a viral extraction tube with viral lysis buffer and sterile swab — were manufactured by a South Korean company in Haryana on the outskirts of the city. Today, 40 per cent of all tests being carried out in the city are RATs, show Delhi government figures.

Shah’s meeting with Kejriwal and the introduction of RATs on a large scale seem to have done wonders for Delhi in spite of rising coronavirus cases. The city’s test positivity rate — positive Covid-19 results as a proportion of total tests carried out during a period — cooled off from 37 per cent on June 13 to 12.8 per cent on June 30. While the number of daily tests in Delhi has grown nearly three times to cross 20,000 on many days, the number of coronavirus cases added by the city daily appears to have risen by just about a third during the same period.

Amid all this, the Delhi High Court on June 22 asked the Kejriwal government to further increase RAT testing. Delhi is now planning door-to-door screening in containment zones, taking RAT to all parts of the national capital in July.

Even if Delhi’s test positivity rate has declined sharply, it continues to be quite high — 1.5 times the national average on June 30. While it earlier had a higher positivity rate than Maharashtra (where Mumbai and Pune account for the bulk of cases being added daily), Delhi now seems to have fewer people testing positive per test. However, it should be noted that the Maharashtra government had not allowed RATs — where results can be obtained in half an hour for severely ill patients, pregnant women and those with co-morbidities — until June 23.

RATs check if tested persons have the presence of antigens (a type of proteins expressed by an infection) in their respiratory tract and confirm whether or not they are Covid-19-positive, within 30 minutes of the test. However, the World Health Organization (WHO) has stated that such rapid tests could detect antigens only when the virus in replicating actively — in the event of either early or acute infection. WHO clearly said in April that these tests might miss “half or more of Covid-19 infected patients… depending on the group of patients tested”. That implies RATs could not detect the virus in many cases even if the person being tested were infected and had the potential to spread the disease to hundreds others.

Even the Indian Council of Medical Research (ICMR) considers Reverse Transcription Polymerase Chain Reaction (RT-PCR) as the gold standard for Covid-19 testing. It stated earlier that any person testing negative in RAT should be “definitely tested sequentially by RT-PCR”.

What, however, seem to make a case for RATs in India are their quick turnout of results and lower cost involved, especially given the size of the country’s population. “We have a large population and RT-PCRs require skilled technicians and specialised labs for molecular tests... We have done rapid testing for Human Immunodeficiency Virus (HIV) in the past. I don’t see any harm in doing the same for Covid-19 as well,” says Dr Sabina Khan of Delhi-based Hamdard Institute of Medical Sciences.

“You have to think like a policy maker and not just an epidemiologist. If you compare it with RT-PCR, the sensitivity in rapid testing is definitely compromised. But you cannot do RT-PCR in densely populated areas of India. It is too expensive to be done across India; we cannot afford it. If the objective is to test more people, the logistics for RT-PCR are too cumbersome. Rapid testing is the only way forward,” says a senior official in the health ministry.

Dr Jayashreee Bhattacharjee, a former principal of Vardhaman Mahavir Medical College at Delhi’s Safdarjung Hospital, says: “We do rapid testing with pregnancy and heart ailments. Students have to write exams. People have to get back to work. Rapid testing may give many false negatives. But they save a lot of time and money as we test for a rapidly spreading infection in a highly populated country.”

Business Standard tried to reach Delhi Health Minister Satyender Jain over phone but had not been able to get through to him as of the time of publishing of this report. His comments would be incorporated as and when received.