How to Tell if You've Got a Negative or Positive Lateral Flow Test

Taking a rapid antigen test but are unsure of how to read it? Find out how to differentiate between a negative and positive lateral flow test.

Dr Alasdair Scott MBBS FRCS PhD

February 10th, 2022

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Since the outbreak of the coronavirus pandemic, the mass rollout of lateral flow testing has enabled us to limit the spread of COVID-19 by identifying quickly and accurately whether someone is highly contagious. A positive lateral flow test indicates that the person taking the test is emitting COVID-19 particles and should self-isolate immediately to reduce the risk of passing the virus on. With 1 in 3 people likely to be asymptomatic if infected with the virus, lateral flow tests, or rapid antigen tests as they’re also known, have been invaluable in identifying contagious individuals.

When it comes to taking a lateral flow test, all tests come with detailed and accurate instructions, to help you successfully take a sample and perform the test yourself. However, it can be difficult to grasp the process – especially if it’s new to you. So for those looking for more information and guidance as to what constitutes a positive lateral flow test, we’re going to break down a) the science behind taking a rapid antigen test and b) how to tell if you’ve got a negative or positive test result.

How Do Lateral Flow Tests Work?

Many of us have become accustomed to taking lateral flow tests before attending an event or seeing loved ones. However, a lot of people do so without really understanding how the test works. In this section, we’ll look at how a rapid flow test functions in determining whether there is COVID-19 in a sample or not – and what a positive lateral flow test result looks like versus a negative one.

Lateral flow tests work in a similar fashion to pregnancy tests, in that they’re able to detect a specific compound or hormone. In this case, the compound that the test is designed to detect is the coronavirus antigen, or protein that makes up part of the coronavirus.

Woman taking rapid antigen lateral flow test

It’s able to do this by taking a diluted sample of mucus from someone’s nose and/or throat and dropping it onto the testing device. The sample is then pulled along the strip in the middle of the device via capillary action to small clusters of antibodies. These antibodies, similar to the ones our bodies develop after having a vaccination or a coronavirus infection, detect coronavirus antigens in the sample.

Lateral flow tests are helpful for testing widely in communities and offer insights into those who may be asymptomatic but still carrying the virus. Even someone who feels physically fine may be unknowingly carrying and passing on coronavirus to the people they come in contact with. When people have high viral loads and are contagious, a lateral flow test can indicate the presence of Covid in their body and help them to not spread it to others.

What a Negative Lateral Flow Test Looks Like

If your test comes back negative, you’ll know because there’ll only be one line on the test device at the “C” position. This means that no coronavirus antigens were detected in your sample and it’s unlikely that you were infected with the virus at the time of testing. You can see an example of a negative lateral flow test picture below.

What a Positive Lateral Flow Test Looks Like

If there are coronavirus antigens in the sample you’ve taken, the rapid flow test will show a positive test result – indicated by two lines on the test at the “C” and “T” positions. Whether the lines are faint or strong, two lines indicates a positive result. So, even a faint positive test result shows that you are likely to be infectious with coronavirus and should self-isolate immediately – regardless of whether you’re showing symptoms or not.

You can see a positive lateral flow test picture below:

Picture of positive rapid antigen lateral flow test

From the positive lateral flow test picture above, you can see that two lines have appeared on the testing device at the “C” and “T” positions, which indicates that the person who has taken the test is infectious with coronavirus.

Picture of negative rapid antigen lateral flow test

Are Lateral Flow Tests Accurate?

Lateral flow tests are readily available for individuals to carry out themselves at home and give quick results. The fact that anyone can take this kind of COVID test makes some people assume that they aren’t accurate at detecting coronavirus in a sample. But just because lateral flow tests can be carried out by anyone in their home doesn’t mean that they aren’t an effective and reliable method of testing for coronavirus.

Much research has been carried out on the accuracy of rapid antigen testing and the results confirm that they are indeed reliable tests. A lateral flow test is highly accurate at detecting the presence of coronavirus in people who have high levels of the virus and are likely to pass it on to others. However, a lateral flow test is less good at detecting coronavirus in people who have low levels of the virus but these people are also less likely to be infectious to other.

This means that if you have a positive result then it’s highly likely that you have coronvirus, even if you have no symptoms. To remind you, classic symptoms fever, cough, change in taste / smell, muscle aches and tiredness.

Recent data released by the UK Government reinforced the accuracy of lateral flow tests and showed that fewer than one false positive result occurs for every 1000 lateral flow tests taken.

On the other hand, because lateral flow tests are not so accurate when people have low levels of the virus, a negative result is not perfect in ruling out coronavirus. Data suggests that for every 100 people with coronavirus detected by highly sensitive polymerase chain reaction (PCR) tests, about three may have an incorrect “false” negative result with a lateral flow test.

Can You Have a Positive Lateral Flow Test but Negative PCR result?

With two types of testing available, it can be possible to do both tests simultaneously and get different results. This happens because of the different abilities of each test. As we’ve already covered, both PCR and lateral flow tests are highly accurate, but the lateral flow test is not as good as PCR tests at picking up low levels of virus in a sample.

PCR tests are highly sensitive and can detect coronavirus in a sample even before a person starts becoming contagious and can even continue to detect the virus in the days, weeks and months after a person has physically recovered and is no longer infectious.

Because PCR tests can detect coronavirus at a lower level than lateral flow tests, it’s unlikely that you’ll test positive on a lateral flow test but negative on a PCR test. This is why the UK government doesn’t require positive lateral flow tests to be confirmed by PCR.

However, it’s not uncommon to get a negative lateral flow test but a positive PCR test. This is because individuals who have just caught coronavirus or are recovering often have low levels of the virus that can’t be detected by lateral flow but can be detected by PCR. So if you take both a PCR and lateral flow test early or late in the infection, you may get different results.

The graph below from the BBC News summarises the differences between the two tests:

PCR vs. antigen test sensitivity

What Happens After Getting a Positive Lateral Flow Test?

Because the rapid antigen test is most accurate at picking up coronavirus when you’re highly infectious, if you do have a positive lateral flow test result, it’s extremely likely that you have the virus. In the event that you do test positive on a lateral flow test, you should start isolating immediately to reduce the chance of you spreading the virus to others.

Woman isolating after getting a positive rapid antigen lateral flow test

It’s no longer compulsory to take a follow up PCR test if you have a positive lateral flow test result – you can start isolating as soon as you test positive.

It’s also a good idea to let anyone you may have come into contact with know that you’ve tested positive, so they can start lateral flow testing – or do a PCR test if they prefer.

Order Lateral Flow Tests Today with C19 Testing

Taking regular lateral flow tests is important for helping to manage the spread of COVID-19 and keeping the people we care about safe. When you test before attending an event or visiting a vulnerable person, you can reduce your chances of passing on the virus and give yourself peace of mind.

We offer highly accurate and reliable lateral flow tests here at C19 Testing, and you can order yours today. Our lateral flow tests provide accurate results, quickly, and feature a nose-only swab to make it easy to check whether you’re infectious with coronavirus.

Enjoy free express delivery on all orders placed before 8pm on weekdays and 6pm on weekends. Order through a trusted, Government-listed testing provider now.

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