Rapid antigen tests, better known as RATs, have become an important tool in Australia’s arsenal against COVID-19.
While PCR tests are still available — and remain the “gold standard” for testing — many of us have turned to rapid tests out of convenience or as part of a requirement to return to work or school.
RATs can provide results within minutes, don’t require a healthcare worker or lab to process, and help to catch COVID-19 cases that otherwise would have gone undetected.
But they also have their limitations: they’re less accurate, cost money (unlike PCRs, which are free), and can provide false negative or false positive results.
So, the next time you reach for a RAT, here are a few things to keep in mind.
When do rapid antigen tests work best?
Rapid antigen tests are designed to detect antigens found on the surface of the virus (PCR tests, on the other hand, look for the presence of viral genetic material).
They’re most accurate when people have symptoms, and are generally best used within the first seven days from when symptoms appear.
“If people are symptomatic, they often have more virus present in their nose or mouth … and so rapid tests perform better,” said infectious diseases physician and microbiologist Paul Griffin.
Dr Griffin said RATs were most effective at picking up the disease when people were in their infectious phase — when they are more likely to be shedding the virus — which is why they can be a useful screening tool.
But he said testing negative on a RAT wasn’t a guarantee you couldn’t spread COVID-19, though “the probability of being able to transmit it is certainly lower.”
Because rapid antigen tests work best when viral load is at its highest, they are less reliable at picking up COVID-19 in the very early or very late stages of an infection.
“It takes a while for the viral load to get up to the point where it’s detectable,” said registered nurse and disease control expert Thea van de Mortel from Griffith University.
Preliminary research suggests RATs are unable to detect SARS-CoV-2 until at least two days after someone is exposed to the virus, and that it takes, on average, three days to test positive.
Professor van de Mortel said if people had a known exposure to SARS-CoV-2, it was a good idea for them to wait a couple of days before taking a rapid test.
“The other thing you can do is test two to three days in a row after you start testing — to improve the chance that you pick it up,” she said.
Which rapid tests are most effective?
For a rapid antigen test to be approved in Australia, it must meet certain performance requirements outlined by our national drug regulator, the Therapeutic Goods Administration (TGA).
This includes having a clinical sensitivity of at least 80 per cent, and a clinical specificity of at least 98 per cent.
To understand what that means in practice, take this example from Dr Ashwin Swaminathan:
“Let’s imagine we line up 100 people who definitely have COVID infections. Applying a RAT kit with 80 per cent sensitivity would lead to a positive result in 80 people — with 20 receiving a “false negative” result.
“If we line up 100 people who are definitely free of COVID infection, using a RAT test with 98 per cent specificity would mean that 98 would test negative — and two people would have a “false positive”.”
The TGA classifies rapid antigen tests into three categories: acceptable sensitivity (greater than 80 per cent), high sensitivity (greater than 90 per cent), and very high sensitivity (greater than 95 per cent).
Dr Griffin said although there was “a lot of variability”, RATs that require a nasal swab tended to be a bit more sensitive than RATs that test saliva, because “typically we find higher amounts of virus in the nose”.
“Of course, there’s benefits in having a variety of tests … for example, the saliva-based tests are often more appropriate for kids because they’re easy to administer,” he said.
Whichever test you use, Professor van de Mortel said it was important to follow the instructions carefully, and — if it’s a saliva swab — avoid eating, drinking, smoking or brushing your teeth in the 10-30 minutes beforehand.
Do rapid antigen tests detect Omicron?
When Omicron emerged in late 2021, there was some concern that rapid antigen tests may have a reduced sensitivity when detecting the new variant.
That’s because mutations on the virus can sometimes alter the structure of viral proteins, meaning test kits may no longer be able to detect them.
But Dr Griffin said those early concerns hadn’t been proven with good quality evidence.
“We’ve seen lots of rapid antigen tests be positive in recent times for BA1 and BA2 … so it remains likely that they perform similar to how they perform with previous variants,” he said.
The TGA is currently reviewing and reporting the evidence provided for rapid tests and how sensitive they are to emerging COVID-19 variants, including Omicron.
“As each variant of concern emerges, the manufacturer is expected to undertake analysis … to verify that their tests continue to perform as intended,” the TGA says.
“Many of the variants have a prevalence of mutations in the spike protein of the virus rather than the nucleocapsid protein, which is the predominant target of these kinds of tests.”
As of mid-March, 85 per cent of antigen and rapid antigen tests had shown “evidence of performance” against Omicron, one test (a point-of-care test administered by health workers) had been suspended, and a handful were still under review.
Preliminary research published in December by Australian scientists found 10 commercial RAT kits had comparable sensitivity for detecting Delta and Omicron variants.
My RAT says I’m negative, but I feel unwell?
If you’re feeling unwell but your rapid antigen test returns a negative result, it’s advised that you isolate for another 24 hours and do another RAT, or get a PCR.
“At the moment, with such high rates of transmission, if you have potentially compatible symptoms … you should treat yourself as positive until you get a negative PCR,” Dr Griffin said.
“Rapid antigen tests come with a significant rate of false negatives.”
It could be that you collected the sample too early or incorrectly, that your immune system is working hard to keep your viral load very low (even though you’re infected), or that the test simply isn’t sensitive enough. It could also be that you have another illness.
If you receive a negative PCR test, it is unlikely that you have COVID-19.
But Professor van de Mortel said you should continue to stay isolated until your symptoms resolve, to protect other people from whatever illness you may have.
“One of the interesting things about this period is that our flu cases have gone through the floor, as have most upper respiratory viruses, because of the precautions people are taking,” she said.
“Personally, I’ve moved to a mindset that says: if I’m feeling unwell, even if it’s not COVID, I should try to stay home … and if there’s something I absolutely have to go out for, then I would be wearing a mask to protect others around me.”
It’s also important to remember that if you are a close contact, you must stay in isolation for seven days, regardless of a test result.
My RAT says I’m positive, but I feel fine?
The risk of a “false positive” result is much lower than a “false negative” one — remember, approved RATs must have a specificity of 98 per cent.
It’s also not uncommon to experience very mild or no symptoms at all when infected with COVID-19.
That being said, if you haven’t left the house in weeks or a positive result is very unexpected, you may wish to repeat the test 24 to 48 hours later, or have a PCR test done — but make sure you stay isolated until you have a negative result.
For COVID-positive people coming to the end of their seven-day isolation period, no RAT test is required to leave — however, it’s worth remembering that there is a small chance you might still be infectious.
If you have symptoms like a sore throat, runny nose, cough or shortness of breath (in the last 24 hours), you should stay isolated until 24 hours after your symptoms have resolved.
Even with no symptoms, Dr Griffin said it’s a good idea to mask up and avoid high-risk settings for a few additional days.
“In some states there’s a rule to wear a mask for an additional period of time following isolation, and that’s another really good way of reducing the chance of people who still might be slightly infectious of passing it on,” he said.