Antigen tests: when to take them, and how to know you've done them correctly

Antigen tests could be key to keeping your loved ones safe from Covid-19 this Christmas. But are you using them correctly?

When is the best time to take an antigen test if you're meeting with someone?

Open any social media this week and you’re sure to see many people sharing their antigen test results in a photo. It’s the latest sign of general worry around Christmas gatherings amid the spread of Covid-19, with many people taking daily tests to ensure their loved ones’ safety at Christmas.

As more of us turn to the shop-bought tests to ease our anxious minds, here we share expert advice on the best practises when taking an antigen test as well as the best time to take one before you meet with anyone else.

When is it appropriate to take an antigen test and when should I book a PCR test?

An antigen test is a useful tool for anyone who is not presenting any symptoms of Covid-19 and who is feeling well. It helps to detect the virus in asymptomatic people to prevent its transmission among the community. It is ideal for school-going children or college students, or anyone who is regularly in high-risk environments: these include cinemas, theatres or concerts, bars or restaurants, multi-household visits, car sharing with people from other households, and participating in contact sports.

An antigen test card. Picture: Sam Boal / RollingNews.ie

“Antigen tests are very specific, meaning that a positive test almost certainly means you have Covid-19,” Cork doctor Dr Doireann O’Leary says in an Instagram post.

“However, they’re not very sensitive meaning they can miss up to 50% of positive cases; a single antigen test will not detect a significant proportion of people who would be identified by PCR as infected and potentially infectious for others.” 

Dr O’Leary advises doing an antigen test “before any gathering eg. before meeting family for Christmas dinner or friends for lunch.” 

Antigen tests are not to be used by those presenting with symptoms of Covid-19 (eg: cough, tiredness, etc). For anyone who suspects they may have Covid-19, they should self-isolate and arrange a free PCR test online or through a GP. If you get a positive result on an antigen test, you should also self-isolate and arrange a free PCR test.

“An antigen test is quicker but less reliable than a PCR test. In some cases, it may not detect the virus. This is why you should do them regularly over a period of time,” the HSE advises.

How do I take an antigen test correctly?

Follow the instructions that came with the test you bought as these may vary from test to test. Dr O’Leary has shared a useful video on how to correctly self-test using an antigen test.

 

 The general steps involved, according to the HSE are:

• Wash or sanitise your hands.
• Put all the items in the test kit on a clean surface.
• Open the tube and place it in the holder - some tubes are prefilled with liquid and some you need to fill with a vial of liquid.
• Blow your nose.
• Open the swab wrapping - only touch the swab at the handle, do not touch the soft pad at the tip.
• Insert the swab into your left nostril until you feel resistance, this is usually about 2cm to 4cm.
• Roll the swab 5 times inside the nostril.
• Using the same swab, repeat this process in your right nostril.
• Insert the swab into the tube until the soft pad is in the liquid.
• Stir the swab around for about 1 minute to make sure the liquid has a chance to absorb the sample on the swab.
• Squeeze the tube with both fingers and gently pull the swab out of the tube.
• Place 1 of the small plastic nozzle caps provided on top of the tube.
• Drop 4 drops of the liquid into the testing device.
• Wait for the recommended time on the instructions leaflet - usually in 15 minutes.
• The result will appear on the test device.
• When you have finished with an antigen test, put it in a plastic bag and put the bag in your bin.

If you have a positive antigen test result, immediately self-isolate and arrange a PCR test. 

I’m meeting a friend for lunch tomorrow. How soon before that should I take an antigen test?

Previously, the HSE’s advice was to take an antigen test twice a week, three days apart as symptoms could take a few days for Covid-19 to appear in your system. This week, medical professionals are encouraging more frequent testing.

If you’re meeting someone from outside your household, do an antigen test as late as you can before you meet them. For example, if you’re meeting for lunch at 1pm, take a test just before you leave home to meet them rather than the morning of or night before.

A viral post on Twitter shows how quickly after a negative result an antigen test can return a positive result.

 

Billy Quilty, an infectious disease epidemiologist at London School of Hygiene and Tropical Medicine, shared a photo of four antigen tests he took within 24 hours. The first was taken in the morning, the second at lunchtime, the third in the evening and the fourth the following morning.

His first and second tests were negative while a faint line can be seen next to the third test. The fourth test was a clear positive result. He stressed that people should do antigen tests just before meeting up with anyone.