What is an antigen test?
There are two types of testing for Covid-19, or SARS-CoV-2. The polymerase chain reaction (PCR) test is the most accurate and is the one used by the HSE to determine if a person is positive for the virus. With this test a swab is taken from the back of the nose or throat.
It is then sent to a laboratory which identifies if the virus is present at a genetic level. This requires specialist equipment and is time-consuming but has a high degree of accuracy.
Antigen tests are much quicker - results can be back within 30 minutes. The swab taken from the nose or throat is put into a solution that will release proteins from the virus. That protein can then be detected by a change of colour on an indicator.
Which is more accurate?
The PCR undoubtedly. When the Covid-19 pandemic emerged in 2020 several countries (notably the US and the UK) spent billions of euros buying antigen testing kits to allow people to quickly identify the virus.
But there were sharp discrepancies in accuracy between different manufactures with some correctly identifying only a small minority of cases.
The experiment was quickly abandoned. Since then, the more expensive PCR tests, which typically take 24 hours to produce a result, have become the standard.
Does antigen testing have a place in Ireland?
This has been the subject of an ongoing debate and has divided opinion among scientists and doctors, as well as within the Government apparatus. The Government commissioned an expert group, chaired by professor Mark Ferguson of Science Foundation Ireland, to assess if antigen testing had a place.
The six-person group was split and two of its members dissented from the report .
The group concluded that rapid tests, including antigen testing, did have a place and complement PCR testing programmes.
“The evidence base for deployment of rapid tests designed for use in asymptomatic populations is growing as are the numbers and types of commercially available rapid tests. It is important that Ireland is positioned to take advantage of these developments.”
So it recommends manufacturers who have met the required European standards for antigen testing. Critically, it also said that self-testing could be considered using nasal swabs or saliva, as many tests are now easy to use with easy-to-follow instructions.
So in what circumstances can it be used?
The group said it could be deployed in workplaces, nursing homes, sports and other events, schools and colleges. Indeed the Government sees it as a tool in the wider reopening of society. Regular antigen testing for workers, people going to events, students in college, sports spectators, residents in hotels, will help identify at least some of those who have symptoms.
Already there are pilot projects underway where workers in meat plants get regular testing. There are also pilot testing in four universities and some employers like the Kerry Group have been using them.
The group was particularly concerned about younger age groups.
“Particular focus should be paid to young people who will be the last to be vaccinated, who are more likely to be asymptomatic, have been identified as key, early drivers of new waves of SARS-CoV-2 infection, have the lowest risk of severe disease and the greatest desire to socialise responsibly.”
It recommended twice-a-week testing for example on third-level campuses.
And are there drawbacks?
Yes there are. Antigen testing is less accurate than PCR and accuracy tends to drop when there is low prevalence. The higher the viral load, the more accurate it tends to be. The test is not as sensitive as PCRS so can throw up false positive results, and also false negative results. A review carried out by the respected Cochrane Library of 64 study reports on antigen concluded that 72 per cent of people with symptoms were correctly identified as having Covid-19, whereas that fell to 58 per cent in people with no symptoms.
There is a particular problem with false negatives, in that the person who gets a negative test might assume they do not have the virus when in fact they might have it but are asymptomatic or presymptomatic.
So where do the fault-lines lie?
There is a reluctance within the HSE and in the National Public Health Emergency Team to embrace antigen testing to the extent recommended by the expert group. Conversely, politicians want it to be widely used for aviation, hospitality, third-level colleges (Simon Harris has championed this) and sports matches (Jim O’Callaghan has been advocating this).
A layer of complication has been added by the decision by Lidl to sell off-the-shelf antigen tests.
For chief medical officer Tony Holohan this spell trouble. He has constantly referred to the test being only about 50 per cent accurate.
“For every two cases of the disease where an antigen test is done, it will miss one of these cases. We can’t have people behaving as if they don’t have the disease when they do,” he said on Monday.
Basically, he is saying that an individual will take a home test which shows negative and they think it will give them carte blanche to socialise and mix because they think they do not carry the virus. So in other words, people will begin to give antigen testing a status it does not deserve.
However, for advocates of the antigen test, it is not a replacement for the gold standard PCR test. Its use is not intended for individuals but as part of collective and ongoing screening programmes. It can pick up people who would not otherwise be picked up - which makes it immediately better than having no test, said an expert. If the person gets a false positive, the protocol is that they quickly get a PCR test to confirm the finding. If it turns out that it is a false positive, the most it will mean is one day of self-isolation.
It looks like that view will prevail. As society reopens it is likely that regular antigen testing will join mask-wearing and hand-washing as part of people’s daily lives.
