During the Covid-19 pandemic, authorities in India have been insisting on having a higher share of expensive but more accurate RT-PCR tests over cheaper but less sensitive rapid antigen test (RAT) for plugging holes in detection.

But now a team of researchers from Ashoka University in Sonipat and the National Centre for Biological Sciences (NCBS) in Bengaluru, using computational models, has shown that even a judicious use of rapid antigen tests (RAT) alone could yield good results from epidemiological point of view provided the testing is done on scale.

The paper authored by Philip Cherian and Gautam Menon of Ashoka University and Sudeep Krishna of NCBS appeared in the journal PLoS Computational Biology on Thursday.

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The scientists, however, insisted on a few conditions. Firstly, RAT should be of reasonable sensitivity, much higher proportion of people should be tested (around 0.5 per cent of population per day), those who undergone testes should be isolated till the results are available and testing should be accompanied with other non-pharmaceutical interventions such as wearing mask and keeping physical distancing, etc.

More tests during peak

“At the peak of the pandemic, we should be carrying out five times more (RAT) tests than what we are doing today. That is about 8 to 9 million tests per day. But when the cases come down, on an average, you can test less,” Menon told BusinessLine.

While RT-PCR tests are more sensitive than rapid antigen tests, they are more expensive and do not provide results immediately. Therefore, the precise mix of tests needed to optimise outcomes while accounting for cost constraints has been unclear.

During the Covid pandemic, different States in India have been using different mixes of RT-PCR and RAT. The increasing reliance on lesser-sensitive RAT – as they are much cheaper than RT-PCR -- by many States was a bone of a contention between them and the Union Health Ministry.

Their analysis suggested that use of only rapid antigen tests could achieve similar outcomes, in terms identification of total infections, as using only RT-PCR tests -- as long as the number of people tested is high enough. This suggests that governments in lower and middle-income countries might be able to achieve optimal outcomes by concentrating on ramping up testing using less sensitive tests which provide immediate results, rather than favouring RT-PCR.

The authors suggested that governments should continue to explore different mixes of tests. . Given that the costs of testing are falling, this mix could also be recalibrated regularly to monitor what makes the most economic sense.

“Tests are continually improving, and the trade-offs are in favour of rapid testing, even if it is less sensitive,” Menon said. “Modeling the effects of using different combinations of tests, keeping in mind their relative costs, can suggest specific policy changes that will have a substantial effect on changing the trajectory of the epidemic.”