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Q&A: How rapid antigen tests differ from PCR tests – and why you must use them appropriately

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Ronan Glynn, deputy chief medical officer. Photo: Gareth Chaney/Collins Photos Dublin

Ronan Glynn, deputy chief medical officer. Photo: Gareth Chaney/Collins Photos Dublin

Ronan Glynn, deputy chief medical officer. Photo: Gareth Chaney/Collins Photos Dublin

We were told yesterday that around 300,000 people with possible Covid-19 symptoms last week used a rapid antigen test in a bid to find out if they had the virus.

About 200,000 of these got a “not detected” result and they did not self-isolate or go for the gold standard PCR test at a HSE centre.

Deputy chief medical officer Dr Ronan Glynn has warned about the inappropriate use of antigen tests.

At the same time the Government is going to subsidise their cost and people are turning to antigen testing because HSE PCR test slots are booked up.

There is a lot of confusion around testing at a time when Covid-19 is spreading at dangerous levels.


What is a PCR Covid-19 test?

A PCR test for Covid-19 is seen as the most accurate. It detects genetic material from the virus and is regarded as very sensitive. It can only be performed professionally by HSE staff or at a private centre. A sample is taken from the nose with a swab and processed in a laboratory, with a result about a day later.

It is the test anyone who thinks they could have symptoms of Covid-19 should take. Clearly many people are following this advice. A record 26,000 were tested in one day earlier this week, with the demand leading to delays in getting an appointment in many counties.


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How does a rapid antigen test differ?

A rapid antigen test aims to detect proteins from the virus that causes Covid-19. It can be done at home by getting a sample from the nose, with a result in around 15 minutes.

It is not as sensitive or reliable as a PCR test. It can be affected by timing of the infection, and if people are doing the test themselves they may not get enough of a specimen from their nose.

Anyone who has possible symptoms of Covid-19 should not rely on a rapid antigen test.

However, the expert group on antigen testing set up by Health Minister Stephen Donnelly said the test could potentially identify people who have no symptoms but are infectious.

But they cautioned that antigen tests “may not always pick up if you have been infected with the virus”.


If I get a negative PCR test but still have symptoms, am I in the clear?

No, you must stay home until 48 hours after the symptoms are gone.


So what is the key advice when deciding which test to use?

If you have any symptoms, self-isolate and book a PCR test. If you have no symptoms and may be going to visit someone or going to a bar or restaurant, you could use an antigen test. If an antigen test is negative, you must still follow protective measures.


So when can I use an antigen test, and are they of much value?

If you have no symptoms and are going to be mixing with people, you could take an antigen test beforehand. Although it won’t pick up every infection, it will detect the virus in some people who might otherwise have gone on to infect others.

If it is positive, you need to self-isolate and book a PCR test.

Good advice for people who are mingling with others outside their home or are going to the pub, in a car share or are involved in contact sports, is to take two tests a week. Hence the need to make them more affordable. The test acts as a precaution, although again the test must be done properly at home and has limited reliability.


What if I am fully vaccinated and a close contact of a member of the household who has had a positive PCR test?

The HSE will send you a packet of antigen tests. You must restrict your movements for five days. The reason for this is because the risk of transmission in households is higher. If you are still negative after that time, you can go back to normal.


What happens with school children with no symptoms who are close contacts, from next week?

The parents will be given a packet of rapid antigen tests for their child. However, the child can continue to attend school unless one of the tests is positive. Schools are regarded as lower risk.


What role do antigen tests have in the workplace?

Many employers have been using them on staff a few times a week. Again, they may not find everyone with the virus but they will identify some and stop them from infecting colleagues, thereby reducing absenteeism.


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