Before the government started using rapid antigen kits, between 20,000 and 24,000 Reverse Transcription-Polymerase Chain Reaction tests were done every day.
However, the numbers have come down to 9,000-12,000, especially over the last two days, after the state government began to aggressively push rapid antigen tests.
These tests, known to deliver quick results, are less accurate with a sensitivity rate of 50.5% to 84%.
The more expensive RT-PCR test is considered the gold standard in detecting Covid-19 infections.
Of the 42,458 tests done on August 3, 29,488 (69%) were rapid antigen tests, while 12,970 (30%) tests were performed using RT-PCR kits.
A day prior to that, of 27,989 tests, 18,074 (64%) were on antigen and 9,915 (35%) were RT-PCR tests.
“Rapid antigen test became the focus and target driven, reducing the number of RT-PCR tests. However, RT-PCR is being increased now,” a government official said, adding that though total tests are up, rapid antigen tests show considerable false negatives. A positive report in an antigen test is considered definitive, but a negative report will be further referred for an RT-PCR test if the person is symptomatic.
About 24% of the samples referred for retesting on RT-PCR have turned out positive in the state.
Officials said the government had not issued any advisory to reduce RT-PCR tests but has only asked for the rational use of test kits.
District officials told ET that there were certain gaps in referring symptomatic people with negative rapid antigen reports to retest using RT-PCR. “Either the staff who collect samples do not maintain a clear record of people who are symptomatic and asymptomatic, or people do not show up for retesting,” a district official said.
Dr CN Manjunath, in-charge of lab testing in Karnataka, said the state will make it mandatory to collect samples for both rapid antigen and RT-PCR testing at one go. “So, if the person is symptomatic and the test comes negative on antigen, samples can be immediately sent for the RT-PCR test,” he said. On the decrease seen in RT-PCR tests, he said “it is the right strategy.”
Dr R Giridhar Babu, ICMR task force member on surveillance and research of Covid-19, said the focus should be on increasing the tests. “Bengaluru is testing 9,000-10,000 samples a day, of which 2,000 people test positive. The positivity rate in the city is high. We should test 10 times more to ensure we are not missing out on cases,” he said.