Having symptoms of fever, cold or cough and getting a rapid antigen self-test for COVID-19 done at home? Then, doctors say certain factors need to be understood, and more importantly, a negative result has to be interpreted clinically.
The rise in COVID-19 infections over the past few weeks has made a number of persons opt for home testing using Rapid Antigen Test (RAT) kits.
“RAT is ideal to find out if one has COVID-19, quickly . But how the results are interpreted is important. If a person is negative, it does not mean he/she has no COVID-19. When compared to RTPCR, which is the gold standard and has sensitivity of 80 to 90%, RAT has a sensitivity of 40%. A high viral load is required for the test,” Suresh Kumar, consultant Infectious Diseases, Apollo Hospitals, said.
It was also important to understand that if a person tests positive in the RAT, he/she is infectious to other people, he said. RAT is used in other circumstances as well. “A person can end isolation after repeating RAT on day 5 and 7 and is negative. They will not transmit the infection. Surgeons get a RAT done before procedures. In RAT, the turnaround is fast and it is cheaper,” he said.
But it has certain drawbacks, he said, adding: “In the last one month, we have had a lot of people who got a RAT done and went around as the result was negative. Even those who were positive did not isolate themselves. So, if a person is positive, they have to understand that they are infectious, need to isolate and wear a mask. With a variety of home tests available now, there is no accountability as well.”
“If a person is negative, we have clinical suspicions and look for positivity in family members, any travel history and if they have typical symptoms of COVID-19. In such circumstances, PCR is the confirmatory test,” he further said.
C. S. Sripriya, associate professor of Microbiology, Rajiv Gandhi Government General Hospital, said that RATs do not pick up infections as well as an RTPCR. “If a person is negative, he/she has to confirm through a RTPCR test. But we are seeing things are actually the other way around in the community. Those who are positive are getting RTPCR test done for confirmation,” she said.
If a person tests negative but has symptoms, he/she should definitely get a RTPCR test done. Now, the RTPCR testing facilities are available across the State and the report is available on the same day, she added.
Srinivas Rajagopala, senior consultant, Interventional Pulmonology and Sleep Medicine, Kauvery Hospital, said different kits have different sensitivities — with the range being 40 to 70%, he said.
The timing of RAT is important. “When one is symptomatic, it is useful to test within the first five to seven days. If tested beyond that period, the test sensitivity will further come down,” he said.
If asymptomatic, there can be two situations — had an exposure to somebody with known COVID-19, and no known exposure but want to know status. ‘“If you had a point exposure, you need to wait for five to seven days after exposure for the test to be meaningful. If you had the exposure yesterday and you tested today, and the result is negative, it means nothing…You may not have sufficient load of virus in the body for the test to be positive. One test does not rule out if one has COVID-19 or not…Repeating the test in a gap of three to seven days would be more sensible,” he said. He said a negative result has to be interpreted clinically.
