OK, hands up — who has a stockpile of COVID rapid antigen tests in the cupboard?
There is a wide variation in the accuracy of COVID rapid antigen tests
Experts say we still need RATs to stop the spread of COVID and ensure timely treatment
There’s a lot you can do to maximise the accuracy of RATs
As COVID continues to do the rounds you may find yourself reaching for one of those tests. But your RAT test could give you a bum steer on whether you are infected with Sars-CoV-2 or not if it has passed its expiry date.
Even if the test is still current – and has been stored at the right temperature – the result may still not be accurate.
While some of the blame for this lies with us not using RATs correctly, a new study suggests manufacturers have also overstated the accuracy of their tests.
So what’s the story, and what can be done to increase test accuracy?
In 2022, a Cochrane review found enormous variation in the ‘sensitivity’ — the ability to accurately pick up a positive case of COVID — between different brands of RATs.
“When tests were used according to manufacturer instructions, average sensitivity by brand ranged from around 34 per cent to 93 per cent,” said Emily MacLean, an epidemiologist at the University of Sydney.
She said RAT accuracy was generally confirmed in the studies by using PCR.
Yet, the World Health Organization recommends RATs have a minimum sensitivity of 80 per cent — which means they pick up at least 80 of 100 positive cases, missing 20.
“So some of those RATs [studied by Cochrane] certainly did not meet WHO’s target sensitivity,” Dr MacLean said.
Dr MacLean and colleagues decided to compare the estimates of accuracy from the Cochrane review with the claims made by manufacturers on the little slips of paper — the ‘instructions for use’ — found inside test kits.
To minimise the impact of ‘user error’, they only included research in which people using the tests stated they followed the manufacturers’ instructions.
The new study of 22 tests, published in Lancet Microbe, found that manufacturers claims over-estimated accuracy, on average, by 12 per cent, meaning an extra 12 positive cases per 100 infected would be missed.
“Not every test is overstating sensitivity but more of them do than don’t,” Dr MacLean said.
‘Real world’ evaluation of tests
While RATs are evaluated before they are sold, the 2022 Cochrane review was looking at how the tests actually worked when they were deployed, Dr MacLean said.
And this is when many factors can affect accuracy.
For example, RATs are known to have lower accuracy in people with few symptoms and in people who test too early after they’ve been infected.
And, as mentioned, failing to follow test instructions can be a problem.
For example, if you eat, smoke, brush your teeth or chew gum before doing a spit test that can throw the accuracy out.
Touching the swab or forgetting to blow your nose before a nasal test can also cause problems.
“Some tests are more robust to user error than others,” Dr MacLean said.
Paul Griffin, director of infectious diseases at Mater Health Services in Brisbane said it was important to understand the limitations of RATs.
“The biggest limitation is the sensitivity. There is a relatively high rate of false negatives, meaning it can fairly easily miss COVID, particularly if it’s done early or incorrectly.”
Dr Griffin said given that the accuracy of tests is generally assessed in “ideal conditions” it’s not surprising that they have lower accuracy in the real world.
However experts suggest medical professionals and people who do tests regularly are more likely to get an accurate result.
‘Tests still important’
While most states don’t collect RAT results anymore, Dr Griffin said the tests still play an important role in controlling the spread of COVID, and ensuring effective treatment is given early enough to work well.
“We still have around 200 outbreaks of COVID in aged care facilities around the country at present,” he said.
“It’s still really important we find people with COVID, particularly those at high risk.”
Dr Griffin said access to oral antivirals was dependent on a positive test, and since PCR tests were now much more difficult to get, this placed more importance on rapid antigen tests.
“While it’s talked about very little publicly these days, COVID is still around and will continue to be for the foreseeable future,” he said, adding everyone should consider using RATs.
“It’s the right thing to do — to isolate if you have COVID.”
His advice for maximising the accuracy of your RAT is to use the Therapeutic Goods Administration list of approved COVID-19 rapid antigen tests to find a test with the highest sensitivity possible, and then be careful how you do the test.
“There are subtle differences in how they all work so you do have to make sure you are following the included manufacturer’s instructions,” Dr Griffin said.
He also says it’s worth purchasing a “combination test” that picks up RSVs and types of influenza.
Call for more transparency in test evaluation
Jon Deeks, a professor of biostatistics at the University of Birmingham, who led the Cochrane review on RAT accuracy, and was co-author on the new study with Dr MacLean, argues for a big-picture change when it comes to test evaluation.
He says there is a lack of transparency in evaluations done by manufacturers and a relatively low level of evidence is used to evaluate tests, compared to drugs.
“We’re about 30 years back in terms of the strength of evidence we can get,” Professor Deeks said.
A spokesperson for the Therapeutic Goods Administration, however, stated the TGA’s expectations for manufacturers aligned with requirements of the World Health Organization (WHO) and the European Commission, and no change was being proposed.
The TGA carries out ongoing independent verification of approved RATs to ensure compliance with requirements.
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