NEW DELHI: A moderate to very high percentage of negative results in antigen tests for Covid-19 are turning out to be positive in RT-PCR testing, making states like Maharashtra treat initial positivity rates with increasing suspicion and prompting others like Tamil Nadu to rely exclusively on the real-time PCR test from the get-go.
Data from two major civic laboratories in Mumbai shows that up to 65% of symptomatic patients who had tested negative for the novel coronavirus in antigen tests were later confirmed positive in RT-PCR testing.
While this underscores the risk of false negatives in rapid antigen testing, Delhi has reported a significantly higher success rate than Maharashtra using the same method. Between June 18 and July 21, around 15% of symptomatic patients who had first tested negative were confirmed positive for the virus in the RT-PCR test.
According to the Union health ministry, over 3.6 lakh samples had been tested in Delhi during that period using the rapid antigen method. Of these, 6% were found positive for Covid-19. Of those that tested negative, 2,294 cases were symptomatic and re-tested using the RT-PCR method for confirmation. Around 15% of these patients were found positive for the virus.
This divergence is the reason why Tamil Nadu, which tests nearly 50,000 people every day across 113 hospitals, relies on RT-PCR as its sole Covid-19 diagnostic tool.
“We have adequate supply of RT-PCR kits and the facilities to run the test. So, we decided to stick to the most reliable test,” state health secretary J Radhakrishnan said. Dr P Umanath, managing director of the Tamil Nadu Medical Services Corporation that procures drugs and equipment for all state-run hospitals, said antigen testing could actually end up delaying intervention because the incidence of false negatives was “too high”.
Bengaluru, a late entrant into the Covid-19 hotspot map, started conducting rapid antigen tests on July 20, totalling a little over 8,500 samples till Friday. No analysis of the test results has been done so far, although there is a consensus among healthcare professionals that antigen testing outcomes cannot be treated as even a remotely correct indicator of the Covid spread.
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“Rapid antigen tests have a lower sensitivity compared to RT-PCR, but the trade-off is that you get the results within an hour. That’s why these tests are used mainly for screening purposes,” said a health official in Delhi.
Delhi conducted 14,810 rapid antigen tests and 5,250 RT-PCR tests in 24 hours last Wednesday. In Hyderabad and other parts of Telangana, 60-70% of all Covid-19 testing is done using the rapid antigen method. Although Telangana has no official data yet on false negatives, health officials have estimated that 7% of these patients go on to test positive in the RT-PCR method.
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