A moderate to very high percentage of negative results in antigen tests for Covid-19 are turning out to be positive in RTPCR 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 Mumbai laboratories show that up to 65% of symptomatic patients who had tested negative for Covid-19 in antigen tests were later confirmed positive in RT-PCR testing, underscoring the risk of false negatives in antigen tests. Between June 18 and July 21, around 15% patients who had first tested negative were confirmed positive for the virus in the RT-PCR test.
‘Antigen testing outcomes not a correct indicator of virus spread’
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.
“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.
Data from two major Mumbai laboratories show that up to 65% of symptomatic patients who had tested negative for Covid-19 in antigen tests were later confirmed positive in RT-PCR testing, underscoring the risk of false negatives in antigen tests. Between June 18 and July 21, around 15% patients who had first tested negative were confirmed positive for the virus in the RT-PCR test.
‘Antigen testing outcomes not a correct indicator of virus spread’
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.
“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.
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