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Explained: Why are Delhi’s Covid-19 testing numbers falling?

In Delhi, the number of RT-PCR tests have come down from an average of around 64,000 per day to an average of around 45,000 per day over the past 6 days.

Written by Mallica Joshi , Edited by Explained Desk | New Delhi |
April 27, 2021 5:35:38 pm
A Delhi Police personnel gets tested for Covid-19 in New Delhi. (Express Photo: Praveen Khanna)

Testing for the novel coronavirus has slumped in Delhi, which at one time boasted of conducting around 1 lakh tests every day.

From testing numbers of 98,957 and 99,230 on April 16 and 17, the city slipped first to 85,000-90,000 daily testing numbers, then to 70,000-odd between April 21 and April 25, to finally crash to just 57,690 tests on April 26.

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Getting a test has become more difficult by the day over the past week, with people trying and failing to book slots repeatedly. Home collection of samples has been all but stopped by most laboratories, who complain of being overbooked and overworked.

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What kind of testing has reduced in Delhi?

Delhi tested 1.14 lakh people on April 10, the highest for the city till date. Close to 74,000 RT-PCR tests were conducted that day; the were the faster but less reliable Rapid Antigen tests.

Over the past week, it is the number of RT-PCR tests — considered the gold standard in testing — that has fallen. This is worrying because there are higher changes of a person who has the infection slipping through the Rapid Antigen test than in an RT-PCR test.

In Delhi, the number of RT-PCR tests have come down from an average of around 64,000 per day to an average of around 45,000 per day over the past 6 days.

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What is the difference between the two tests?

Both tests are done using throat and nasal swabs. Both are supposed to detect the virus in samples.

While Rapid Antigen tests are faster, with a result possible in around 30 minutes, RT-PCR tests take at least an hour-and-a-half to two hours to produce a result, and the actual time taken to deliver a report is almost always much more.

The sensitivity of the Rapid Antigen test (sensitivity is the ability of a test to correctly identify diseased individuals, calculated as the number of positives detected out of the number of diseased individuals tested) ranges ranges from 50 per cent to 85 per cent, depending on the viral load of the patient.

The sensitivity of the RT-PCR test is much higher, reporting as little as 2 per cent false negatives — that is, a sensitivity of up to 98 per cent.

Most of the Rapid Antigen tests are being conducted for fast detection in markets, bus stands, railway stations, etc. by government officials.

But why has Delhi begun to suddenly test less?

On April 17, Delhi Chief Minister Arvind Kejriwal said that many private labs were picking up more samples than their capacity, and as a result, test reports were delayed.

Calling the delay “criminal”, the CM said that it could mean the difference between life and death for some patients and ordered action against labs that continue to pick up more samples than what they could handle.

As cases have surged dramatically, many labs have said a lot of their staff are down with Covid-19, and they are stretched too thin to do a large number of tests daily. The virtual end of home collection of late has contributed to the lower testing numbers.

Test reports in Delhi are currently available after three days of testing on average; in some cases, the wait time is up to twice that. Sources in the Delhi government, however, said sample collection should pick up again this week, as labs have been able to clear almost all their pending workload. Reports too, should be available within two days, they said.

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