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Key gap in antigen testing protocol prompts rethink

On Thursday, the Delhi government directed all districts in the Capital to ensure no lapses take place in testing people with symptoms of the coronavirus disease (Covid-19) through the Reverse Transcription-Polymerase Chain Reaction procedure, considered the most accurate method of detecting the viral infection.

Updated: Sep 25, 2020, 00:07 IST

By Sweta Goswami, Hindustan Times New Delhi

While experts suggest that if RATs must be used, those symptomatic individuals who test negative must undergo a RT-PCR test (or another accurate molecular test), this is not foolproof either -- at least 40% of those infected with Sars-Cov2 are asymptomatic. (HT Photo)

An analysis of Delhi’s Covid-19 testing data by the health department showed that, on Wednesday, 1,400 symptomatic people who tested negative in rapid antigen tests (RATs) were not administered the gold standard RT-PCR tests as follow up, highlighting a glaring gap in the Capital’s testing protocol.

On Thursday, the Delhi government directed all districts in the Capital to ensure no lapses take place in testing people with symptoms of the coronavirus disease (Covid-19) through the Reverse Transcription-Polymerase Chain Reaction procedure, considered the most accurate method of detecting the viral infection.

Special secretary (health and family welfare) Udit Prakash Rai issued the directive.

RATs, while fast (results are in within the hour as compared to a minimum of 24 hours for RT-PCR ones), have a high propensity to give out false negatives -- which means they identify an infected person as uninfected. HT has repeatedly stressed that these tests should be used only in certain contexts (such as in containment zones, or in airports or railway stations), but many states and Delhi continue to use them. Uttar Pradesh and Bihar, for instance, once laggards in testing, have conducted millions of tests, many of them RATs. Delhi too continues to depend heavily on them. In the week to Thursday, RATs accounted for 81.5% of the total 386,924 tests conducted in the Capital.



While experts suggest that if RATs must be used, those symptomatic individuals who test negative must undergo a RT-PCR test (or another accurate molecular test), this is not foolproof either -- at least 40% of those infected with Sars-Cov2 are asymptomatic.

“Total 1,437 antigen negative symptomatic persons have not undergone RT-PCR. Kindly ensure that this is made NIL from today (Thursday). This means from today there should be no antigen negative symptomatic case who is not tested for RT-PCR,” read Rai’s communication, sent to all district magistrates (DMs) and chief district medical officers (CDMOs).

HT could not immediately ascertain if similar data is available for more days.

The directive also had a district-wise break-up of the number of cases recorded on Wednesday of symptomatic people who tested negative in rapid antigen tests and whose samples were not tested subsequently through RT-PCR.

According to the data, north-west Delhi, which also has the maximum number of active Covid-19 cases (over 5,000) in the city, had 870 cases on Wednesday alone where symptomatic antigen negatives were not followed up with an RT-PCR test.

North-west Delhi was followed by the south-west district, where 258 symptomatic persons were not administered RT-PCR tests after testing negative for Covid-19 in rapid antigen tests.

“I guess more than 80% of all tests being conducted in Delhi everyday are rapid antigen tests. Heavy reliance on antigen tests is never advisable,” said Dr Lalit Kant, former head of epidemiology and infectious disease at the Indian Council of Medical Research. “Rapid antigen test is only a test of choice for routine surveillance in containment zones and screening at the point of entry. In all other situations, RT-PCR tests should be given priority. Merely increasing the testing through rapid antigen tests will not give a true picture of Delhi’s real positivity rate.”

A Delhi government spokesperson confirmed that a specific direction was issued by the state Directorate General of Health Services (DGHS) to all field teams conducting rapid antigen tests that there should be no lapse in conducting follow-up RT-PCR tests.

“Medical officers of each of the 11 districts have been asked to strictly ensure that no symptomatic RAT negative is let go without an RT-PCR test. The DGHS has issued necessary directions in this regard,” the spokesperson said.

In fact, the medical officer has been asked to go ahead with an RT-PCR procedure without necessarily having to wait for the result of the antigen test.

“Earlier the field teams used to wait until the results came and if negative many people would just leave the centre immediately, despite officials asking them to wait. Now symptomatic people can be tested with RT-PCR right there without waiting for the antigen results,” the spokesperson said.

A senior health department official said on condition of anonymity that the Delhi government is also working on a plan to increase the number of RT-PCR testing laboratories. The official added that the Delhi government is awaiting approvals from the Union government on the plan.

“Daily RT-PCR tests have now crossed the 10,000 mark, while the existing combined capacity of all the 54 labs in Delhi is around 11,000 tests per day. Modalities are being worked out to scale up RT-PCR sample processing capacity further. It is in various approval stage from the Central government,” said the official. To be sure, Delhi still doesn’t do more than 10,000 RT-PCR tests every day. In the week to Thursday, for instance, this level has been breached four times.

Sandeep Mishra, district magistrate of the north-west district, the one with the highest case load, said his administration was now ensuring that access to an RT-PCR test is as easy in the district as to the rapid antigen test. “We are ensuring [that] both RAT and RT-PCR facility should be available at all testing centres.”

BM Mishra, DM of south district, said: “They (health officials) do not let people leave the test centres at any cost until the RAT results come. As soon as the result comes negative on RAT, a fresh sample for RT-PCR is also collected from the symptomatic person.”

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