Coronavirus: Rapid antigen testing misses almost half of positive cases - Kiwi study

Rapid antigen tests (RATs) are nowhere near as accurate as traditional RT-PCR tests when it comes to detecting the virus behind COVID-19 but they have their place, Kiwi scientists behind a new study say.

The Government announced on Thursday an end to its year-and-a-half-long ban on point-of-care RATs. Associate MInister of Health Ayesha Verrall said while we were trying to eliminate the virus "the cost of missing a case was too high", so the Government wanted every test to be via the ultra-accurate but slow RT-PCR (reverse transcription polymerase chain reaction, or PCR), a type of nucleic acid amplification test (NAAT).

But now we're resigned to living with the virus in our midst, "benefits such as accessibility, convenience and speed" need to be taken into account, she said. Many countries overseas have been using RATs - which can give results in 15 minutes - since last year. Some have also adopted point-of-care NAATs that give results almost as quickly.  

Kiwi researchers wanted to see how well a range of them worked in the New Zealand context, so put them to the test - seeing how five different RATs and three NAATs performed compared to traditional PCR.

They found the RATs could miss up to 44.3 percent of COVID-19 infections, their research - published in the New Zealand Medical Journal on Friday - revealed.

"Performance was variable dependent on the product," said study senior author Gary McAuliffe, an Auckland-based microbiologist, who noted other brands they didn't bother testing might "perform worse". 

"The advantages were an answer in 20 minutes, they are very useful for quick decision-making, and they are very suitable for testing people in hospitals to help decide on patient management pathways. There are definite advantages to rapid antigen tests in our current situation as resource constraints are less of an issue compared with PCR, particularly if an individual is able to do their own testing."

The problem is RATs struggle to pick up infections in their early stages, even if people are already infectious. They rely on there being a large quantity of viral proteins, which build up over the course of an infection - making the tests more accurate the closer people are to being symptomatic. 

None of the five RATs looked at in the study - made by Abbott, AccessBio, Siemens, Innova and Roche - met the World Health Organization's standard of 80 percent. 

Rapid NAATs much better, but have drawbacks too

NAATs work by amplifying genetic material that might only be present in minute quantities, well before symptoms start to show.

Traditionally, small portions of each sample are pooled and a test done - if it comes back positive, the individual samples are tested to find out who has the virus. This allows "huge numbers" of people to be tested at once, but at high cost - about $200 per test, according to the University of Otago

Two of the three rapid NAATs tested in the study use a similar but slightly different technology to RT-PCR called isothermal amplification. They missed about 12 percent of cases detected by RT-PCR. One NAAT - Roche's Cobas Liat, which used RT-PCR and gives a result in 20 minutes - was just as accurate as lab-based RT-PCR testing. 

The Cobas Liat is already being used in some New Zealand hospitals, but they can only perform one test at a time, which the researchers said "considerably limits throughput", with Dr Verrall saying the Government plans to increase labs' daily capacity to 60,000 RT-PCR tests a day.

They're also in short supply and expensive - costing more than $10,000 - but would "suit remote settings (eg. rural hospitals without molecular laboratories) when rapid results are needed".

Study co-author James Ussher, of the University of Otago's Department of Microbiology and Immunology said reducing the strain on labs was "necessary" as more and more Kiwis contract the virus. While Dr Verall said all suspected cases will still need to be confirmed via a lab-checked RT-PCR test, Dr McAuliffe says RATs "could replace a significant proportion of asymptomatic testing currently being performed by PCR in the community", easing the load.

Politicians welcome RATs, but maybe 'don't fully understand them' 

National and the ACT Party, which have been pushing for the unbanning of RATs for much of the pandemic, welcomed Dr Verrall's decision to allow them from December. ACT leader David Seymour said the ban "held us back as a country", while National's COVID-19 spokesperson Chris Bishop said on Twitter it was "a shame" it took this long. He didn't respond to Newshub's requests for comment.

But rapid tests aren't a gamechanger, one expert told Newshub. 

"People are pushing RATs because they don't fully understand them," Anne Wyllie, a Kiwi microbiologist based in the US behind a different kind of PCR test, SalivaDirect, told Newshub. 

"There is certainly a time and a place for them, but this has to be properly considered - and properly explained. People need to be fully aware that RATs only guarantee their negative status for a few hours max - they cannot even guarantee a full 12-24 hours so you have to be careful how you're assessing your risk and how you're acting as a result of the negative test if you are potentially at risk."

While people waiting for the results of an RT-PCR test are generally aware they should isolate until they've got the all-clear, overseas rapid tests have been used to check people's infections while out and about in the community. Kiwi entrepreneur Sir Ian Taylor told The AM Show earlier this month how he recently had to undergo rapid testing before entering the Fox studio lot in California. 

"At least RATs can give you a quick answer in a time of need - particularly if you think you are positive or have symptoms," said Dr Wyllie. "Testing is so inaccessible in NZ at present and the waits are too long - perhaps it will help to ease pressure on labs to help capacity for other necessary testing."

Anne Wyllie.
Anne Wyllie. Photo credit: Anne Wyllie/Twitter

Dr Wyllie said many Kiwis might be "drawn to RATs as they avoid the deep nasopharyngeal swab and can also mean they don't have to wait for hours for testing". 

She said there has been a lot of "miscommunication" about the different types of test around, even amongst Members of Parliament. SalivaDirect has been picked up in the US, and used in the recent NBA season, because of its superior accuracy to RATs and low cost - around $1 per test.

"While it's faster and more streamlined, it's not exactly rapid. Though at the same time, you can do 92 samples or so in three-four hours, which is more than testing on the Liat… Granted, we don't have the same level of sensitivity as the Liat..., but we're still on par with many other great PCR tests for a fraction of the cost, increased turnaround time, and flexibility." 

Darryl Carpenter, group manager of COVID-19 testing and supply at the Ministry of Health, told Newshub they were "aware of rapid PCR tests and are monitoring the research into them, including this study".

"Rapid PCR tests are already used in some laboratories in New Zealand. The ministry's priority is to safeguard the health and wellbeing of all New Zealanders and continues to keep abreast of developments in science, public health thinking, along with changes in the virus to ensure our approach to testing for COVID-19 is effective. We regularly review emerging testing options to make sure we are continually adapting our response."

Carpenter said traditional RT-PCR tests "will continue to be used as the primary diagnostic test, but this will be supplemented by saliva-based PCR testing, rapid antigen testing and rapid PCR tests". 

The Ubiquitome instrument that has been developed with support from MBIE is capable of analysing up to 16 samples at a time. This platform will be evaluated by the ministry to determine its use case to best support testing through the new point of care testing evaluation framework that commenced on November 15, 2021."

It can provide results inside an hour from samples obtained via swab up the nose or Dr Wyllie's SalivaDirect, but has two constraints - it's currently "constrained by the issue of short-supplied reagents" - a substance added to the test to cause a chemical reaction - and isn't a point-of-care test, requiring "very good laboratory practice principles to operate".