Guidance

Understanding lateral flow antigen testing for people without symptoms

An explanation of the technology behind asymptomatic testing and the role these tests play in the national coronavirus (COVID-19) testing programme.

NHS Test and Trace is making rapid lateral flow antigen testing available alongside standard lab-based polymerase chain reaction (PCR) tests. These tests play a different, but crucial role in the fight against COVID-19.

Around 1 in 3 individuals with COVID-19 do not display symptoms. Opening testing up to catch those showing no symptoms will help to find positive cases earlier and to break hidden chains of transmission.

Lateral flow antigen testing

Lateral flow testing is a fast and simple way to test people who do not have symptoms of COVID-19, but who may still be spreading the virus. In some circumstances lateral flow tests can also be used for other diagnostic tasks.

The tests are easy to use and give results in 30 minutes. Those who test positive must immediately self-isolate to avoid passing the virus on to others.

Why lateral flow tests are being offered

Around 1 in 3 people with COVID-19 do not have symptoms, so a test that rapidly detects these otherwise hidden cases is a very useful additional tool for tackling the virus.

Lateral flow tests are practical, easy to interpret and can be used in a wide range of settings. This makes them ideal for widespread use in the community.

Clinical evaluation by Public Health England (PHE) and Oxford University shows that the tests perform best when levels of virus are at their highest. Thousands of positive COVID-19 cases have already been detected using these tests.

Each positive case identified can help prevent many additional people becoming infected over time.

How lateral flow testing works

An introduction to lateral flow testing

Lateral flow is an established technology, adapted to detect proteins (antigens) that are present when a person has COVID-19. The best-known example of a lateral flow test is the home pregnancy test kit.

The test kit is a hand-held device with an absorbent pad at one end and a reading window at the other. Inside the device is a strip of test paper that changes colour in the presence of COVID-19 proteins (antigens).

Test results

Negative result: one line next to C shows the test is negative.

Void: no lines, or one line next to T, means the test is void. You will need to re-take with a fresh test kit.

Positive result: two lines, one next to C and one next to T, even faint lines, shows the test is positive. You or your test provider must report this test result to the NHS.

If you get a positive result you must self-isolate immediately, preventing further transmission of the virus.

Rapid lateral tests at a test site

If you took a rapid lateral flow test at a test site and the result was positive:

Continue to self-isolate until you get the result of the PCR test. Follow the advice you’re given when you get the result.

In England, if you do not take a PCR test within the next 2 days of a positive rapid lateral flow test, you must self-isolate for 10 full days even if you get a negative PCR result.

Find out how long to self-isolate on NHS.UK.

Rapid lateral test at home

If you did a rapid lateral flow test at home and the result was positive:

Continue to self-isolate until you get the result of the PCR test. Follow the advice you’re given when you get the result.

A negative result means that active COVID-19 infection was not detected, at the time of the test. However, this does not guarantee that you do not have COVID-19. You should continue to follow COVID-19 rules, including regular hand washing, social distancing and using face coverings where required.

How to take the test

Taking a lateral flow test usually involves taking a sample from the back of the throat near the tonsils and from the nose, using a swab.

The swab is dipped into an extraction solution. This is then dripped on to the device's paper pad, producing the reaction that gives the result.

The result will be visible on the device precisely 30 minutes after the sample is applied. Unlike a PCR test, there is no need to send the sample to a lab.

Who can be tested

You can get a free rapid lateral flow test twice a week. Find out about testing in Scotland, Wales or Northern Ireland.

If you do not have symptoms, you can:

Find out about regular rapid tests if you do not have symptoms on NHS.UK.

If you do have symptoms of COVID-19, you should not visit an asymptomatic test site. You should self-isolate immediately and follow the guidelines:

Get a PCR test here to confirm if you have COVID-19 when the following apply:

  • you have a high temperature
  • you have a new, continuous cough
  • you’ve lost your sense of smell or taste or it has changed

Effectiveness of lateral flow antigen testing

Lateral flow tests can help to drive down the spread of COVID-19.

Lateral flow tests deliver a rapid result, in 30 minutes. They can find positive cases with high levels of virus that are easy to transmit to others, helping to intercept and reduce further infections.

Lateral flow tests used by the UK government go through a rigorous evaluation by the country’s leading scientists. This means they are accurate, reliable and successfully identify those with COVID-19 who don’t show symptoms and could spread the virus without realising.

Lateral flow tests are not being offered in isolation, but alongside a range of other measures to drive down cases and break chains of transmission. See more on this topic on the Public Health England blog.

Sensitivity of the tests

‘Sensitivity’ refers to the proportion of people with COVID-19 that have a positive test.

When a person has low levels of virus in their system, lateral flow tests are less sensitive than some of the other tests we use, such as PCR tests which we mainly use for people with symptoms.

When levels of virus are at their highest and people are most likely to pass on the disease, lateral flow tests can detect the vast majority of cases.

PCR and lateral flow have different roles to play in controlling the virus, so it isn’t helpful to directly compare them in terms of how sensitive they are:

  • Lateral flow is useful for finding out if a person is infectious now, and able to transmit the virus to others. The level of sensitivity is high enough to detect the vast majority of these cases. Lateral flow testing is less likely to return a positive result outside the infectious window.

  • PCR is useful for confirming a suspected case of COVID-19, where the person is already self-isolating and is showing symptoms. Higher sensitivity of PCR means it can identify genetic material from COVID-19 even after the active infection has passed.

The different levels of sensitivity are therefore appropriate for the ways they are used.

How lateral flow testing was trialled

The tests underwent a rigorous validation process including evaluations from Public Health England and the University of Oxford. The Medicines and Healthcare products Regulatory Agency (MHRA) provides ongoing regulatory oversight.

Trials were carried out on the tests in a wide range of environments.

Large-scale pilots have also been carried out, including the whole city pilot in Liverpool in November 2020. As a result of the pilot, 897 positive individuals who would not otherwise have known they were infected, tested positive using lateral flow tests.

Pilots and evaluations

We have an ongoing programme of piloting and evaluating other potential uses of lateral flow tests in a number of institutions and workplaces, such as Jaguar Land Rover and John Lewis Partnership.

This includes piloting the regular testing of contacts of people who tested positive for COVID-19 in order to find more COVID-19 cases and break the chains of transmission, as people who have had close contact with a confirmed positive case are at higher risk than normal of having the virus.

A recent study suggests that regular testing contacts of confirmed cases is an effective way to reduce transmission of COVID-19, and could potentially reduce the need for self-isolation for contacts of positive cases

Published 27 January 2021
Last updated 23 April 2021 + show all updates
  1. Updated to merge 'find a pharmacy where you can collect rapid tests' and 'find a test site where you can collect rapid tests' to 'find where to collect test kits or get a test at a test site' in 'Who can be tested' section.

  2. Updated guidance on what to do following a positive rapid lateral flow test, including when to self-isolate. Updated guidance on how to get regular rapid tests if you do not have symptoms.

  3. Updated to add video 'Introduction to Rapid Lateral Flow testing'.

  4. Added a link to find out if your area offers rapid asymptomatic lateral flow testing.

  5. Added a new section on pilots and evaluations.

  6. First published.