Covid-19: RT-PCR tests at a high, positivity rate crosses 6% in Delhi

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The Capital on Thursday set a new record for the number of RT-PCR tests performed in a single day as the city reported over 3,000 new cases of coronavirus disease (Covid-19) for the third day in a row.

With the number of Reverse Transcription-Polymerase Chain Reaction (RT-PCR) tests going up, the daily positivity rate also started rising — 6.23%, the highest in 18 days. Experts said this was because these tests have a low chance of missing positive cases, and thus provides a better idea of on-the-ground spread of the virus. Rapid antigen tests, which the Delhi government favours, tend to throw up “false negatives” that can skew the data.

Of the 55,891 total samples tested in the last 24 hours according to Thursday’s bulletin, 13,985 were RT-PCR, considered by experts and authorities as the “gold-standard” for Covid-19 tests. This means that a quarter of all tests conducted were RT-PCR, one of the highest proportions ever recorded in the city.

Delhi bettered a record set a day earlier (12,596 tests) as it continued to improve the share of the more accurate test. On average, 23.2% of all the tests conducted in the last week have been RT-PCR, against 19.2% from the week before – highlighting how the share has been increasing.

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In terms of the absolute number of RT-PCR tests conducted, on average 11,528 tests were conducted during the last seven days, as compared to 9,254 tests the week before – again highlighting how the number of RT-PCR tests have been rising.

The alternate rapid antigen tests are cheaper, easier to use in the field, and give results in just 15 minutes. However, they are not nearly as precise at molecular tests. The Delhi high court had earlier asked the Delhi government why the 14,000 capacity of RT PCR tests in Delhi was not being utilised as the rapid antigen tests were only 60% “accurate”.

While Thursday’s positivity rate went past the 6%. mark, an average positivity rate of 5.6% was reported during the last seven days. The positivity rate is indicative of the transmission of disease in an area, with a positivity rate of 5% or below sustained over two weeks suggesting that the infection is in control, according to WHO.

Experts are concerned that the spike in cases must come under some measure of control before the festive season and as winter approaches because more mingling and colder weather feed the virus.

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“The 5% positivity rate is also indicative of whether the number of tests being performed is enough. Naturally, if you do more RT-PCR tests, you are likely to pick up more cases because it is more sensitive. However, if you increase the numbers using rapid antigen test, the positivity rate is likely to come down but the government would be picking up fewer cases. The rapid antigen tests are supposed to be used for screening people in containment zones and not for diagnosing someone in general,” said Dr Rajinder K Dhamija, professor of medicine and head of department of neurology at Lady Hardinge Medical College.

On Thursday, Delhi also reported 26 deaths due to the infection, pushing the city’s total Covid-19 toll closer to 6,000. The number of deaths is fewer than the average 37 deaths being reported in the city daily since mid-September.

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