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.

However, at least 348 infected people did not test positive by this point-of-care method that can give results within 15 minutes. Only 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 nearly 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.

Hence, the testing protocol states that any symptomatic person who tests negative has to be retested. There are chances, however, of some asymptomatic positive cases being missed.

“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 rapid antigen tests; more than 50% of the tests are done using this method,” said Singh.

Others, however, said that 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 the 20 people might have been missed if the testing had not been scaled up. Testing and isolating people early on helps reduce the further spread of the infection,” said Dr Shobha Broor, former head of the department microbiology at the All India Institute of Medical Sciences.

In Delhi, initially, the tests were restricted to people living in containment zones, then expanded to hospitals, and are now available in dispensaries as well.

The kits come with a sterile swab, an extraction tube, a viral lysis buffer to break open the cells for the test, and a Covid-19 antigen test device. Once the sample is collected, it has to be placed in the extraction tube that inactivates the virus reducing the bio-safety requirements. Once the sample is mixed properly, it is stable only for an hour. The cap of the extraction tube has to be replaced by a nozzle and 2-3 drops of the solution have to be put in the test well. The test can be interpreted between 15 and 30 minutes.

Delhi cases

Delhi recorded 1,349 fresh Covid-19 cases on Tuesday, taking the city’s tally to over 1.25 lakh, while the death toll mounted to 3,690, authorities said.

After reporting new cases in the range of 1,000 to 2,000 for nine consecutive days, fresh infections in the national capital had dipped to 954 on Monday.

Twenty-seven fatalities were recorded in the last 24 hours, according to a Delhi health department bulletin issued on Tuesday.

The number of active cases stood at 15,288, marginally up from 15,166 on Monday.

The national capital had reported its highest single-day spike of 3,947 on June 23.

The Tuesday bulletin said the death toll due to Covid-19 has risen to 3,690 and the total number of cases have mounted to 1,25,096.

The death toll stood at 3,663 on Monday.