Rapid antigen tests ideal for coronavirus screening in the community

While rapid antigen tests used repeatedly in the same population can be used for screening, RT-PCR tests are designed for use with symptomatic people and so need not be low-cost but require high analytic sensitivity to return a definitive clinical diagnosis given a single opportunity to test.

With novel coronavirus cases accelerating or plateauing across the world, the focus should shift from using a test that has high sensitivity to pick even small amounts of the virus, such as the molecular test (RT-PCR), to rapid antigen tests which can detect infections, a Perspective piece published in The New England Journal of Medicine says. This message has been misconstrued on social media without understanding the context in which the researchers from Harvard T.H. Chan School of Public Health, Boston and the University of Colorado, Boulder have advocated this shift.

In the Perspective piece published in NEJM, the researchers have focussed on which test to be used as a screening tool in a community or the entire population in the U.S. to quickly identify infected people, including those who are asymptomatic, and isolate such people so as to quickly cut the transmission chain. Rapid antigen tests are not only quick (can return results in 15-30 minutes compared with 24-48 hours for RT-PCR) but are cheap and do not rely on laboratory and expensive equipment to carry out testing.

But rapid antigen tests have low sensitivity and therefore produce false negative results half the time. It is for this reason that rapid antigen test should be used repeatedly in the same population as part of an overall testing strategy. Frequent testing thus makes up for poor sensitivity. Based on modelling, Daniel Larremore, an applied mathematician at the University of Colorado, Boulder and his colleagues found that testing people every three days ad isolating those who test positive could prevent 88% of virus spread.

No State in India has so far been screening whole communities or populations with either rapid antigen tests or RT-PCR to diagnose those who have been infected. Instead, States follow syndromic surveillance where testing is carried out on people who exhibit fever or any of the other COVID-19 symptoms. Instead of asking States to repeat the tests in those who have a negative result with a rapid antigen test, even in its June 23 advisory, ICMR has asked States to validate all negative results from rapid antigen tests with a RT-PCR. 

The NEJM authors too make a distinction between tests used on people showing symptoms and for screening a population, and bats for RT-PCR for clinical tests. “Clinical tests are designed for use with symptomatic people, do not need to be low-cost, and require high analytic sensitivity to return a definitive clinical diagnosis given a single opportunity to test,” they write.

Using RT-PCR to screen a community or population has several problems. RT-PCT test is expensive compared with a rapid antigen test making repeated testing on the same individual every few days even more expensive, takes at least one-two days to test and a couple of days to inform the individuals of the test result. The slow turnaround time would mean that infected individuals will come in contact with others and spread the virus even while waiting for the test result. Also, it may pick people who have been infected but are no longer infectious. This might be a problem when such people have to be isolated even when they no longer spread the virus.

Published in The Hindu on October 3, 2020