Positivity rate of rapid antigen tests in Delhi: 6.33%

nNEW DELHI: Around 6.33% of the people tested using rapid antigen kits in Delhi were infected with Covid-19, according to the data by the union health ministry. A total of 336,172 people were tested using the method between June 18—when it was rolled out in Delhi—and July 21, said Rajesh Bhushan, officer on special duty, health ministry, in a press briefing on Tuesday.

At least 348 infected people did not test positive by this point-ofcare method that can give results within 15 minutes. About 0.7% of people who exhibited symptoms similar to the coronavirus disease (Covid-19) were re-tested using the gold standard RT-PCR test.

“Of those who were found negative, 2,294 symptomatic people underwent an RT-PCR test. And, 348 or about 15% of these samples tested positive,” said Bhushan.

The test is highly specific—99.3% to 100%—meaning anyone who tests positive using the kit definitely has the infection. However, with a lower sensitivity—50.6% to 84%, depending on the viral load—the test may throw up a false negative for people who might have the infection.

“What this shows is that the rapid antigen test is not as sensitive as the RT-PCR test; it is rapid and scalable but cannot replace RT-PCR. Having said that, states where a higher number of cases are being reported cannot depend solely on the time-consuming RT-PCR, as the turnaround time would be very long and there would be delays in measures to be taken. Now, even pooling of samples [a method where tissue sample pools are created by pooling a certain number of randomly selected tissue samples (of the same population) together] isn’t helping as most pools are testing positive,” said Amit Singh, associate professor, Centre for Infectious Disease Research, Indian Institute of Science, Bengaluru.

“To check the results on field, the government can try testing via RT-PCR tests and rapid antigen method simultaneously for some samples to see how many positive cases are being missed. In Delhi, the decline in the number of cases coincides with the picking up of the antigen tests; over 50% of the tests are done using this method,” said Singh.

Others said quickly detecting even some of the positive cases can help reduce the total burden of the infection. “The rapid antigen tests are used to quickly test a huge number of people, which would never have been possible with just an RT-PCR test. Now, for example, if 20 persons among a 1,000 are missed out, a successive RT-PCR will detect, say, 17 of these cases. Even if we miss the three people, we are still doing better because all 20 people might have been missed if the testing had not been scaled up,” said Dr Shobha Broor, former head of microbiology department, AIIMS.