What Is an Antigen Test? The Ultimate Guide

If you want to travel or check you aren’t infected with COVID-19, take a rapid antigen lateral flow test at home. Learn more and order yours today.

Dr Alasdair Scott MBBS FRCS PhD

February 23rd, 2022

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It’s likely that at some point in the last two years you, or someone you know, has taken a rapid antigen test or lateral flow test as they’re also known. After being widely used throughout the pandemic to further understand how coronavirus spreads, these tests are now being used in a more limited capacity. However, there is a lot of different information surrounding rapid antigen lateral flow tests and it can be difficult to know what’s accurate and what’s not.

We’ve written a detailed guide to antigen testing, including:

  • What is an antigen test?
  • How a rapid antigen lateral flow test works
  • The accuracy of antigen tests
  • How to do a rapid antigen lateral flow test at home

So, what is an antigen test?

Simply put, a rapid antigen lateral flow test detects small parts of the coronavirus (called antigens) which can be present in a swab or saliva sample. It can identify if someone is infected with coronavirus and may be able to pass it onto others.

This kind of testing has been used throughout the pandemic to determine how the virus spreads and to help people safeguard themselves and others from being exposed to infectious individuals.

A rapid antigen lateral flow test

The process of taking a rapid antigen lateral flow test is generally straightforward. It enables people to do a test in their homes or workplaces as opposed to travelling to an external testing facility.

How rapid antigen lateral flow tests have helped to slow the spread of COVID-19

One of the many reasons that coronavirus has spread so quickly is because around one in three people who have the virus don’t display any symptoms, like fever or cough. Although these people have no symptoms, they still carry and can pass on the virus. Because these people don’t feel sick, they’ll go about their day-to-day lives as usual – potentially spreading the virus to others who will get sick. Identifying people who have coronavirus, but don’t have any symptoms is a crucial step in stopping the spread of this virus – which is where rapid antigen lateral flow tests come in.

As we’ve already covered, this type of test can identify the presence of COVID-19 antigens in a sample of mucus or saliva at a stage where someone is highly contagious with coronavirus – regardless of whether they’re showing symptoms.

Because these tests are low-cost, quick and done at home, they can be taken regularly by anyone, helping to pick up infections in people without symptoms – who wouldn’t even know they had the virus.

How does a rapid antigen lateral flow test work?

The actual technology used in most of these tests is formally called ‘lateral flow’, which is why lots of people refer to antigen tests as lateral flow tests. This technology has been used for many years and is actually how home pregnancy tests work.

Rapid antigen lateral flow tests rely on taking a sample from the nose or throat, or a saliva sample which would likely contain coronavirus if a person had the infection. A buffer solution is added to the sample and a few drops of this mixture are placed onto one end of the test ‘cassette’. After 15 to 20 minutes, the result can be read as either positive or negative for coronavirus infection.

If the test identifies coronavirus antigens, it’ll show two lines on the test device, next to both the C and T regions. Even if ta line by the T region is faint, it still means the sample is positive with for coronavirus and the individual should avoid being in close contact with others. For negative tests, there will be only one line present on the device at the C region.

What’s the difference between rapid antigen lateral flow tests and PCR tests?

Rapid antigen lateral flow tests differ from Polymerase Chain Reaction (PCR) tests. PCR tests work by detecting the genetic material (called ‘RNA’) of the coronavirus in a sample, rather than antigens. This requires laboratory equipment and generally takes a few hours. Because PCR tests require specialist equipment and trained staff, they are relatively expensive.

Scientist using pipette

PCR tests are very sensitive and will detect even tiny amounts of genetic material in a sample. This is extremely helpful because a PCR test can tell when someone has the virus – even if they have a very low ‘viral load’ (meaning they don’t have much of it) – and could even be before they’ve developed symptoms. On the other hand, it’s not always useful because PCR tests can remain positive for weeks (or even months) after someone has recovered from coronavirus and is no longer infectious to others. This is because small amounts of genetic material can remain in their nose for a while.

Rapid antigen lateral flow tests can be done at home by anyone, and can identify the presence of COVID-19 antigens in a sample where the person taking the test has a high viral load, or is highly infectious with the virus – regardless of whether they’re showing symptoms or not.

How accurate is an antigen test?

Different brands of rapid antigen lateral flow tests will have different accuracy figures. For example, we currently use a swab test by ACON (ACON Biotech Flowflex SARS-CoV-2 Antigen Rapid Test). Their clinical validation study found that for every 100 positive PCR tests, we’d expect approximately 97 positive antigen tests. This means the sensitivity of the test is 97%.

Overall, rapid antigen lateral flow tests that have passed assessment from Public Health England and the EU are reliable tests and can be trusted to provide an accurate result. Of course, there are limitations to this kind of testing, because they’re most accurate when a person has a high viral load of COVID-19 or is in the highly contagious part of their infection period.

This does mean that even if you’ve only recently been infected with coronavirus and have a low viral load, it’s possible that your rapid antigen lateral flow test will come back negative. However, in this circumstance, it’s unlikely that you’d be contagious to others at the point of taking your rapid antigen lateral flow test.

When would someone need to take a rapid antigen lateral flow test?

As we move away from mass lateral flow testing, there is less widespread use of these tests. However, some circumstances remain where you still need to take a test.

When You Want to Travel

The first instance where a rapid antigen lateral flow test is needed is for travel abroad. The rules surrounding testing requirements for entry or departure from different countries are set by individual governments, so it’s important to check the country you’re travelling to or from. You can visit the Foreign, Commonwealth and Development Office (FCDO) website to see travel advice for each country in the world.

If you’re travelling abroad to a country that does require proof of a negative COVID test, you’ll need to order a rapid antigen lateral flow test kit from a private test provider and do a test in the correct testing window, before your flight. Our antigen test travel calculator can help you work out when is the best time to take your test. After doing so, you’ll be sent a certificate with confirmation of your negative test result and will be able to travel worry-free.

Female passenger wearing face-mask

Certain countries will require a negative PCR test result instead of, or as well as, proof of a negative rapid antigen lateral flow test result, so be sure to check the travel rules for your destination.

Going into Work

While the government will stop offering mass lateral flow tests to the public from 1st April, it doesn’t mean that you won’t need to access one – especially in regards to working.

One of the most widespread measures to stop the spread of coronavirus was to work from home. As a result, countless professionals transitioned to remote working and many still are working this way. Even as people return to the workplace and testing isn’t required, companies still might opt to offer rapid antigen lateral flow test kits to staff, to prevent the spread of COVID-19 in the workplace. If your company chooses to continue offering these tests to staff to allow them to return to the office or workplace, you’ll still need to know how to take the test correctly.

For NHS and healthcare workers, antigen testing will remain a regular part of the work week. Because they’re providing a crucial health service to the public, and in a space where patients infected with COVID-19 may be present, it’s important to take extra precautions to maintain safety. Lateral flow testing allows the spread of coronavirus to be more successfully observed and minimised.

Man wearing face-mask at work

If You Show Symptoms

The public can still access rapid antigen lateral flow tests if they want to test, from providers like us at C19 Testing. Whether you’re visiting a vulnerable loved one or are showing symptoms of the virus and want to know if you have it, you can take a test.

How to do a rapid antigen lateral flow test at home

The process of taking this test consists of the same steps across providers, with some slight variations depending on the brand you’re using. Our guidance for taking a rapid antigen lateral flow test is based on the ACON Biotech Flowflex SARS-CoV-2 Antigen Rapid Test, but it’s important to read the instructions thoroughly before taking your test.

Steps for Taking an Antigen Test

The following steps cover each aspect of the testing process and will ensure that you carry out your test safely and accurately.

You can also watch a video that covers each step of taking a rapid antigen test.

Step 1: Check your rapid antigen lateral flow test kit

Before carrying out the test, it’s crucial to make sure that all the necessary components are in your kit. This includes:

  • Antigen test device
  • Swab
  • Extraction tube with testing liquid inside
  • Extraction tube holder
  • Waste disposal bag
Step 2: Read your rapid antigen lateral flow test instructions

Even if you’re used to taking lateral flow tests, it’s important to always read the instructions before performing the test. Most tests work similarly, but there can be slight differences between brands, including:

  • The time recommended to leave the test to develop. This can vary between 15 and 20 minutes.
  • Whether the kit comes with a separate or built-in lid for the extraction tube.
  • Whether you need to leave your swab in the extraction tube or take it out.
  • How long you need to let the swab sit in the test liquid.
  • Whether you need to take samples from your throat and nose, or just your nose.
Step 3: Blow your nose

Now, you need to ensure your airways are clear. If there is excess mucus in your nose that gets on your swab, it can compromise your sample and result in an inconclusive result. So, it’s recommended that you blow your nose gently before taking your test.

Young woman blowing her nose

Step 4: Sanitise your hands and test area

Before taking your test, wash your hands with water and soap for at least 20 seconds, and clean and dry your table or worktop where you’ll carry out the test too. Doing so maintains the accuracy of your test.

Step 5: Prepare your rapid antigen lateral flow test

You can now assemble all the parts you’ll need to carry out the test. Be sure to do the following:

  • Take the lid off of the extraction tube and place it in the holder provided
  • Place a swab packet on the surface ready to take your sample
  • Open your test device and place it on a flat surface

The letters on the test device refer to the following:

  • C = Control
  • T = Test
  • S = Specimen well
Step 6: Take your sample

It’s now time to take your test. Only take your swab out of the packaging when you’re ready to perform your test, to keep it sterile.

Depending on your test, you’ll need to take your sample from either your nose, or your nose and throat.

For nose and throat tests:

Take the first sample from your throat by circling the swab on your tonsils (or where they would be) around 10 times. Do this on both sides of your throat. You can now take your nasal swab and insert less than 2.5cm into each nostril – moving the swab in small circular motions five times against the inside of your nostril, to gather your sample.

For nose only tests:

A nasal sample is taken by inserting the fabric side of the swab around 2.5cm into your nostril and rotating the swab around five times against the inside of the nostril. Repeat this process for each nostril.

Taking your sample is not supposed to be painful, so if you’re experiencing substantial discomfort, you may be putting the swab too far up your nostril or too far down your throat.

Step 7: Dilute your sample in the test solution

Once you’ve taken your swab, you’ll need to place it into the extraction tube – making sure the fabric side of your swab is submerged in the liquid. Push the swab to the edge of the extraction tube and rotate the swab inside it for around 30 seconds. This helps your sample to mix with the liquid well.

When you take the swab out, squeeze the tip through the plastic tube to squeeze out any remaining liquid. Put the swab in the waste bag provided.

Step 8: Drop your sample onto the test device

Place the lid on the extraction tube securely and swirl the liquid round one more time to make sure it’s well mixed. Now you can squeeze four or five drops of the solution onto the specimen well (indicated by the S on your test) and place the extraction tube in the waste.

Set a timer. After the indicated amount of time on the test instructions (usually between 15 and 20 minutes), you’ll be able to read your result.

Step 9: Dispose of any test waste

You can now put all of your packaging and other materials, such as the extraction tube and swab in the waste bag provided and seal it. Place this bag in the bin.

Step 10: Get your result

The vast majority of rapid antigen lateral flow tests provide results in between 15 and 20 minutes of dropping the test solution onto the testing device. You’ll see that the test starts developing by pulling the test liquid up the test’s central strip. It’s important to give the test the correct amount of time to develop before reading the result.

There are three possible outcomes when you take a test – you could get a positive, negative or inconclusive result. They’re indicated by the following:

  • A negative result shows one line by the C on the device
  • A positive result shows two lines by both the C and T on the device
  • An inconclusive result shows one line by the T on the device

Negative rapid antigen test result

FAQs for Rapid Antigen Lateral Flow Tests

We understand that you may still have questions, so we’ve gathered some of the most common questions in this next section.

Are rapid antigen lateral flow tests accurate?

Yes. This type of test has been confirmed to be a very effective method of identifying if an individual is highly contagious with the coronavirus, and is at risk of passing it onto others around them. When taken correctly, these tests are accurate and reliable – with extremely low fail rates.

Can a lateral flow test tell if I’ve already had COVID-19?

Rapid antigen lateral flow tests are used to work out if someone currently has coronavirus, so no, they can’t tell if you’ve been infected with coronavirus in the past. When someone has had either a vaccine or coronavirus, their bodies produce antibodies in response to the virus. To determine whether you have antibodies, you would need to take an antibody test.

What does an inconclusive test result mean?

An inconclusive lateral flow test means that the the test was unable to determine a positive or negative result. This is usually caused by a fault in the test kit or by not following the instructions exactly. It happens in about 1 in every 2000 rapid antigen tests. If you get this result, you should do another test to obtain an accurate result.

Can I eat before taking a rapid antigen lateral flow test?

Because you’ll be taking swabs from potentially your nose and throat, it’s advised that you don’t eat, drink or smoke for 30 minutes before taking your test.

What happens if my test is positive?

This means that it’s likely that you’re infected with coronavirus and are at risk of passing it on to others. Although it’s no longer a legal requirement to self-isolate when you test positive for the virus, it’s still advised to stay at home until you feel better or start testing negative. If you choose to see your loved ones or go out in public, there is a real risk you’ll pass on the virus.

Accessing a Test

If you’re travelling, you can order a rapid antigen lateral flow pre-departure test from trusted providers like C19 Testing. We’ve done millions of antigen tests for travel and you can get yours here.. We offer free express delivery (when ordered before 8pm) seven days a week and can also provide video supervision if required by your destination country (e.g. USA or Australia).

If you just want some rapid antigen tests to have at home for non-travel use, we also stock those and you can order them here.

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