How do rapid antigen tests work and when should we use them?

By | October 1, 2021

Known as rapid antigen tests, the kits can provide results within 30 minutes. They have been in widespread use elsewhere in the world throughout the pandemic, often available over the counter in pharmacies or supermarkets.

The Netherlands and the UK, for example, have offered free home tests since earlier in the year; Singapore has also just given every household a free pack of testing kits; and in the US, retailers will soon be required to sell rapid tests at cost.

Now Australia is catching up. The national health regulator, the Therapeutic Goods Administration (TGA), is currently completing a thorough assessment of the tests, which will be approved within weeks.

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“As of 1 November, we will have home tests available for self-purchase for people to get on the internet, at pharmacies, convenience stores, whatever channels they want to get,” said TGA head John Skerritt.

“We’re working very actively with a range of companies because the current tests are designed for professional use and they are not home tests.”

In a press conference on Tuesday, Hunt said that these tests are an “important additional protection” for Australians.

“One of the important things is that we can supplement what is known as PCR testing – the testing that we all know if we go to a Commonwealth or a state clinic – with the access to home testing.”

While rapid antigen testing will likely not replace PCR (polymerase chain reaction) tests, supplementing them will be particularly important as states grapple with outbreaks of the highly infectious Delta variant and pathology laboratories struggle to keep up with the high demand for testing.

States and territories would need to establish systems for people to self-report positive results and would then be directed to have a second test at a clinic, according to the TGA.

But what exactly are rapid antigen tests and how are they different from PCR tests?

How do rapid antigen tests work?

In a PCR test, genetic material from the sample, including genetic material from any virus present, is isolated. Then, the sample goes through a thermal process that separates DNA into two pieces of single-stranded DNA. Next, an enzyme called Taq polymerase duplicates the DNA strands. Each of these strands can be used to create two new copies and so on. The cycle is repeated many times to generate millions of exact replicas of the original DNA segment, which allows for identification of the pathogen.

This process can take hours and requires sophisticated lab equipment and specialised technicians, but results are almost 100 per cent accurate in spotting infected people when there is virus in the swab.

In contrast, antigen tests work by mixing the sample with a solution that breaks the virus open and frees specific viral proteins. That combination is then applied to a paper strip that contains an antibody tailored to bind to these proteins if they’re present in the solution. Like a home pregnancy test, the result is reflected as a band on the paper strip.

Samples can be processed on the spot in 15 to 30 minutes. These tests are relatively inexpensive and work best in the early stages of infection, when viral load is highest.

But that speed comes at the cost of sensitivity.

In a March study, researchers from the Doherty Institute in Melbourne tested one of the rapid antigen tests licensed for use in Australia, the PanBio test, manufactured by Abbott.

They tested 2413 people without the virus and 22 with infections confirmed by PCR testing. They found the PanBio test had a specificity of 99.96 per cent, which means that four in 10,000 tests could return a false-positive result.

Even amid the current outbreaks, COVID-19 prevalence rates in Australia remain extremely low. Such specificity could potentially generate more false positives than there are actual positive cases in the community.

However, the major problem was the reduced sensitivity of the test, which could lead to false-negative results. While the test picked up 100 per cent of COVID-19 cases among people who have had symptoms in the previous seven days, its accuracy dropped to only 77 per cent among asymptomatic people, which means that it could miss up to 23 positive cases every 100.

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Should Australia implement rapid antigen tests during outbreaks?

In Australia, the use of rapid antigen tests has not so far been recommended for the diagnosis of COVID-19 because of concerns around their reliability. They have been used at the quarantine facility in Howard Springs, NT, for returnees from India and were trialled earlier in the year in aged-care facilities in Melbourne and Sydney.

Dr Ian Norton, a specialist emergency physician at Respond Global, who oversaw the trial at the Sydney aged-care centre, says rapid antigen tests are most useful when there is substantial community spread. In that case, he says, the aim of testing is to identify people who have high viral load and are most likely to spread the virus. “Rapid tests are very effective at picking up super spreaders. This is where they are most useful,” he says.

“When we’re chasing elimination in Australia, that’s where PCR is the test to use, because we want to catch every single person with the virus.”

But sensitivity is only one marker of a good test, says Norton. Speed, accessibility and cost are crucial traits to consider during outbreaks because, he says, the strategy is more about limiting community transmission than tracing every single case.

Earlier in the year in Sydney, for example, essential workers from three local government areas in the southwest of the city were been asked to get tested every three days for “surveillance testing”.

The policy caused kilometres-long queues at PCR testing centres and returning results took up to 90 hours, which is hugely disruptive. A result that comes when “the horse has bolted” is not useful when trying to control an outbreak, says Norton.

But in a recent study from the National Institutes of Health, researchers found that rapid antigen tests had a sensitivity ( 98 per cent) comparable to PCR for identifying infected individuals if used at least every three days.

Therefore, daily screening using self-swabbing antigen tests done under the supervision of a trained person or via telehealth could keep essential workplaces open and safe, says Norton.

According to the TGA, companies have put in more than 70 expressions of interest for home testing kits to be approved in Australia. 33 of those are already approved for use under the supervision of a health professional, which are being used in some businesses; Westpac, for example, is trialling the tests in a dozen branches in Western Sydney.

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This new announcement means that tests designed for professional use will now be made suitable for home use.

Ideal for border control and preventing outbreaks

Rapid antigen test use would be extremely helpful in high-risk settings such as border control and quarantine, says Professor Mary-Louise McLaws, an epidemiologist at UNSW Sydney and advisor to the World Health Organization.

Currently, international travellers are required to return a negative PCR test 72 hours prior to their flight departure and are tested again on day two after arrival. Those who test positive are then moved to a separate facility.

McLaws says that five-day lag puts travellers, staff and the community at risk. “Returning travellers and quarantine staff should be tested daily with rapid tests,” she says.

“When you lay out two different tests together, they’re very synergistic,” says Norton. He suggests that a PCR test should be done within 72 hours before take-off, then a rapid antigen test at arrival would triage actively infectious cases to be quickly separated from other incomers. Negative, low-risk travellers could be sent to hotel or home quarantine.

McLaws says self-swabbing, take-home rapid tests should be provided to fully vaccinated returnees who have tested negative on arrival, to allow them to home quarantine.

Looking forward, rapid antigen tests could be a valuable tool to prevent lockdowns when a higher percentage of the population will finally be vaccinated. These tests could be used in mass gatherings such as at sports events or music festivals to quickly screen everybody and catch those who are infectious.

“When we have high vaccination rates, we will not be willing to spend millions and millions of dollars on PCR,” says Norton. “Instead, we should be doing a light, rapid test for catching those one or two people who have broken through their vaccine and are now infectious.”

  • This article is published in partnership with Cosmos Magazine. Cosmos is produced by The Royal Institution of Australia to inspire curiosity in the world of science.

Western Advocate – Health