Why is everyone talking about antigen testing?
It started with Lidl selling rapid-test kits off the shelf. That prompted alarm bells among public health officials who started briefing negatively about it. They were concerned that individuals would take a test and get a negative, and would presume they had a free licence to go out and mingle and socialise. But it did not mean they did not have Covid.
So they are not accurate then?
They are accurate, but only at certain times, usually when people are at their most infectious. Overall, various studies have shown the better tests picking up 50-60 per cent of Covid cases, but that rises much higher (depending on the test) when the person is infectious, in other words, when the viral load is highest in the body.
So what's the difference between it and a PCR test?
There are two types of testing for Covid-19, or SarS-CoV-2. The PCR test is the most accurate and is the one used by the HSE to determine if a person is positive for the virus. PCR stands for polymerase chain reaction. 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. It has a high degree of accuracy but typically takes 24 hours to complete. It will identify as positive, for example, a person who has Covid but is no longer infectious.
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 (in the case of coronavirus that will come from its spikes). That protein is detected by a change of colour on an indicator.
So how could antigen testing work here?
It's not a perfect test as it can pick out only about half the cases. But what it is good at is identifying cases that are infectious. So while not great in identifying individuals, it is good as a screening device at places where people congregate: at sports events; in work; at school or college. It can identify those people who are infectious and most likely to spread the disease. And it can do so within minutes. The number of false positives generated has been found to be very low.
So why is it not more widely used?
The Government commissioned an expert group, chaired by Prof Mark Ferguson of Science Foundation Ireland. It concluded rapid tests did have a place and could complement PCR testing.
It found the quality of some tests were good and, critically, said self-testing could be considered. For example, it recommended twice-a-week testing on third-level campuses.
And are there drawbacks?
For individuals, it is not reliable. It can generate false negatives. There are also problems with people not using the self-test kits properly – for example by not taking a proper swab – thus providing an inaccurate result.
So where do the fault-lines lie?
Chief medical officer Tony Holohan has warned about the drawbacks. 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 it in one of these cases. We can’t have people behaving as if they don’t have the disease when they do,” he has said.
However, for advocates of the antigen test, including Government Ministers, it will help to stop new outbreaks when gatherings return. If used efficiently and regularly, it can pick up infectious people who would not otherwise be picked up – immediately.
