Covid antigen tests: Not a ‘true’ positive

With increase in Covid-19 testing, the use of rapid point-of-care antigen tests for diagnosis of the novel coronavirus has also risen.

But not everyone is convinced of the benefits of rapid antigen tests, given its “relatively low sensitivity”.

Diagnostics industry insiders and health workers are not in favour of these tests, especially in regions that have labs capable of offering RT-PCR tests, the “gold standard” for testing Covid.

Asymptomatic persons

“You may as well toss a coin,” says an industry insider on the lack of confidence in rapid antigen tests. And the reason for this can be found in the Indian Council of Medical Research (ICMR) advisory. “A positive test should be considered as a true positive whereas all symptomatic individuals testing negative through the rapid antigen test should be confirmed with a real-time (RT) PCR test,” explains the ICMR.

Doctors question the need for two tests to identify if a patient is Covid-positive, when one RT-PCR test can do the job. Besides, in a false negative reading, the individual is allowed to go home, even though he/she may be asymptomatic or could develop symptoms later. In that time, he/she would have already passed on the infection to those around him/her, they point out. Presently, three kits are approved by the ICMR, of the 23-odd in the fray.

“Lab-based RT-PCR tests are still the gold standard for Covid-19 diagnosis and all other point-of-care testing, including antigen testing, have very limited relevance in diagnosis,” says GSK Velu, Chairman and Managing Director, Trivitron Healthcare.

WHO advisory

In an advisory, the World Health Organization had said the sensitivity of these tests could be “expected to vary from 34 per cent to 80 per cent”, going by the experience with influenza where patients are seen to have “comparable concentrations of influenza virus in respiratory samples as seen in Covid-19..”

Based on this information, the WHO says, “half or more of Covid-19 infected patients might be missed by such tests, depending on the group of patients tested.”

Naturally, questions abound in health circles on why good money is being put behind purchasing rapid antigen tests, when it does not achieve the desired outcome and has to be backed up by an RT-PCR test. In fact, industry watchers urge Governments to give the break-up of RT-PCR and antigen tests, when they put out the daily testing data.

Time and cost benefit

Benefits of rapid antigen tests include the convenience of not requiring additional staff to do the test, a quick turnaround of less than 30 minutes for a result and lower cost, at less than ₹500 a test. RT-PCR tests take about 2-5 hours, require the sample to be transported to a lab and cost₹2,000-odd, depending on the State.

But competition is bringing down these prices as well, claim industry voices, making a distinction between kits, raw materials and the services component of testing. Treatment access activist Leena Menghaney observes that India has a strong RT-PCR network, which contributed to the effective treatment of HIV/AIDS over the last several years.

Anyone with symptoms of Covid should undergo an RT-PCR test, she says.

Rapid antigen tests are useful in remote regions with limited access to regular healthcare facilities. Early testing helps “get to people before they become very sick,” she says, pointing out that a person with a high viral load should be brought in for early treatment.

Diagnostic-lab network Thyrocare Founder Dr A Velumani echoes the observation that antigen tests defeat the purpose of screening, given the false negatives that need to be backed up with an RT-PCR test. The RT-PCR market has collapsed, he observes, from an average 3,000 tests per day in May to 2,000 in June and about 1,500 in July.

“All labs have 25 per cent capacity utilisation,” he says. It is not just about business impact, but antigen tests are proving to be ineffective, he adds.

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