#Open journalism No news is bad news

Your contributions will help us continue to deliver the stories that are important to you

Support The Journal
Dublin: 6°C Wednesday 1 December 2021
Advertisement

Antigen tests: How do they work, when do I use them, and how accurate are they?

A debate is raging yet again on their usage.

This is an extract from a recent edition of The Journal’s coronavirus newsletter, which cuts through the noise and misinformation to give you clear, accessible facts about the coronavirus, Ireland’s fight to contain it, as well as developments further afield. 

This is your one-stop shop for Covid news during a time when it can be hard or overwhelming to try and stay up to date with the latest.

You can read the full edition heresign up to receive the newsletter here or at the bottom of the page. This version contains additional updates.

IMAGINE THE DIFFERENCE a quick, cheap, self-administered Covid test with laser-sharp accuracy would make.

You simply spit in a tube or stick a swab up your nose, and within a few minutes find out whether or not you have Covid – not just an indication one way or the other, but a cast-iron guarantee.

That hasn’t been invented (yet), but we do have rapid antigen tests – quite far from that hypothetical silver bullet, but better than nothing.

These are nothing new but a debate is again raging over their usage.

In a statement yesterday, chief medical officer Dr Tony Holohan urged the public to take heed of the massive pressure facing the health system and to dial back their social contacts.

He also made a point of asking the public not to use an antigen test if they are symptomatic. But why?

Let’s take another broad look at antigen tests. I last gave you this type of overview back in May, but it’s worth revisiting as the landscape is quite different now.

For one, studies have indicated that the tests may be more accurate than first thought.

  • What’s the difference between PCR and antigen?

The gold-standard test is called a real-time polymerase chain reaction test, or RT-PCR and often just PCR for short.

This test looks for the genetic material of the coronavirus, meaning it can be detected in small amounts, even during the early stages of infection. It’s the type used in HSE testing centres.

It’s very accurate – somewhere close to 99% accurate in detecting people who have the virus (known as sensitivity) and >95% accurate in identifying when people don’t have the virus (known as specificity) – but must be processed in a lab, which involves time and money.

Antigen tests (aka lateral flow) look for particular proteins found on the surface of the virus (antigens). They are not quite as accurate as PCR but involve a quicker and less costly process.

covid-19-rapid-antigen-test-kit-on-a-wooden-table-ready-to-be-used-concept-of-a-broad-testing-strategy-to-cope-with-the-pandemic An example of a RADT test kit. Source: Alamy Stock Photo

Both PCR and antigen tests use a swab to take a sample. How this happens varies between the different brands of antigen test, but usually involves at least one of the following: a nasal swab; a throat swab (over the tonsils); or nasopharyngeal (when the swab goes all the way up your nose).

When discussing antigen tests, we are generally referring to rapid antigen diagnostic tests, or RADTs.

  • Why are antigen tests less accurate than PCR tests?

To understand why, you have to understand what a ‘viral load’ is: simply put, this term relates to the amount of virus particles found in the person being tested (for these particular tests, these particles are found in the nose and throat).

Antigen tests work best when the person has a high viral load, generally shortly before or soon after developing symptoms. There is just more material – virus particles – for the test to work with.

This means that antigen tests are not as adept as PCR tests are at detecting cases early. Equally, you could be sick and experiencing symptoms, but a low viral load means Covid won’t be picked up on an antigen test.

A HSE study from earlier this year breaks down the difference in accuracy between the tests well, using real-world examples where people were tested with both PCR and antigen tests:

  • If the person had symptoms and tested strongly positive on PCR (as in, had a high viral load), the antigen test was also positive in at least 9 out of 10 cases 
  • When the person had no symptoms, the antigen test detected Covid-19 in just half (52%) of cases
  • When the person had no symptoms but the corresponding PCR test was strongly positive – as in, the person had a high viral load but no symptoms – 4 out of 5 cases were detected

Antigen samples taken using the nasopharyngeal accuracy were better than nasal swab in picking up the strong positive cases

  • Right, so how are they being used in Ireland right now?

The public is using them widely - one in five people in the past week, to be exact – as they are easily available in shops and chemists.

The HSE also uses them in outbreak settings.

Fully vaccinated close contacts are currently being sent three tests in the post for free.

They are now being rolled out to primary schools - if a child tests positive, the other children in that pod will be sent antigen tests, and if there are two or more confirmed cases among the entire class in a week, all will be sent a test.

People who engage in high-risk activities are also advised to take two antigen tests a week.

  • What is the official public health advice on antigen tests in Ireland right now?

Ireland is a well-documented laggard in Europe when it comes to antigen tests, with NPHET only recently embracing the suggestion that people could self-administer the tests at home.

We also suffer from conflicting advice on their utility – the government is far more in favour of widespread use than NPHET – with no dedicated public health campaign to clear this up.

The HSE currently advises that you take two tests a week (with three days in between) if you are regularly taking part in high-risk activities such as

  • going to cinemas, theatres, concerts, bars or restaurants
  • contact sports
  • multi-household visits
  • car sharing with people from other households

Health authorities in Ireland want the tests to be used as a screening tool rather than a diagnostic tool; they see antigen test as an extra check you can perform after risky activities, rather than a way of checking if that fever and cough is Covid or not.

The advice is very firmly that if you have any Covid symptoms, you should not rely on an antigen test, and instead self-isolate and seek a PCR test.

For example, respiratory syncytial virus (RSV) is a huge problem in children’s hospitals right now, and causes symptoms that overlap with Covid.

An unwell child sent to school with a negative Covid antigen test (by the way, a test is never ‘negative’, it is always simply ‘not detected’) could well be positive for RSV, posing a risk to classmates.

Essentially, if you are going to use an antigen test, the only result that you should heed is a positive one, as a negative result doesn’t ‘green light’ you to behave beyond what the current public health advice is.

  • Why should or shouldn’t we use them more?

#Open journalism No news is bad news Support The Journal

Your contributions will help us continue to deliver the stories that are important to you

Support us now

The tests aren’t perfect, but they are perhaps imperfect in the right way… look, it’s complicated.

Antigen tests miss cases. So does PCR, but to a far, far lesser extent.

Test 100 people with no symptoms but a high viral load, and 20 will come up negative on antigen (that’s for argument’s sake, it’s not quite how the maths works).

On the flip side of that, you may have stopped 80 of the cases who are potentially most likely (through a high viral load) to infect other people – perhaps unknowingly – if none of these people had reason to take a PCR test.

kranj-slovenia-17th-nov-2021-a-student-self-tests-with-a-rapid-covid-19-antigen-self-test-kit-in-a-primary-school-in-kranj-starting-on-wednesday-primary-and-secondary-school-children-in-slovenia A child prepares to take an antigen test at a school in Slovenia. Source: Alamy

The fear among some public health experts here is that they would start to be used as a carte blanche for people to do as they please; the person may have previously erred on the side of caution, but instead now takes a risk because of the negative antigen result.

Or, the person with the negative antigen presumes it’s not Covid once symptoms develop, and do not act accordingly by self-isolating and going for a PCR test.

However, that person is possibly already doing that unspecified activity right now, but without this extra check.

We could be here all day going back and forth.

  • I want some – which ones should I buy?

Make sure it has a CE mark.

All the tests you can buy are approved and tested at an EU level - you can find the full list here, with details of how the accuracy varies between brands, and which ones were rejected for use.

The government is set to move ahead with subsidising these tests.

  • How do I use them?

Carefully, is the best advice. There is significant room for user error. Keep your hands and workspace clean, blow your nose, and avoid eating and drinking half an hour beforehand if you can.

The HSE has an informational video here on how to use their official antigen tests - but! But but but. Read the instructions on the packet you buy.

Now, after all that, I think I’ll go and take an antigen test…

This is an extract from a recent edition of The Journal’s coronavirus newsletter, which cuts through the noise and misinformation to give you clear, accessible facts about the coronavirus, Ireland’s fight to contain it, as well as developments further afield. 

This is your one-stop shop for Covid news during a time when it can be hard or overwhelming to try and stay up to date with the latest.

You can read the full edition heresign up to receive the newsletter here or at the bottom of the page. This version contains additional updates.

About the author:

Nicky Ryan

Read next:

COMMENTS (15)

This is YOUR comments community. Stay civil, stay constructive, stay on topic. Please familiarise yourself with our comments policy here before taking part.
write a comment

    Leave a commentcancel