Questions and Answers about Lateral Flow Tests for Covid-19

Lateral flow tests are now being used by many members of the public. In this blog, I answer some of the common questions that people ask about these tests, which I was asked to comment on for articles for the Guardian Newspaper and VICE UK.

How do lateral flow tests work and how reliable are they?

Lateral flow test (LFTs) work by detecting the proteins on the surface of the SARS-CoV-2 virus that is the cause of Covid-19. They can be carried out at home and don’t require a laboratory to process the test, unlike PCR tests. They also give a result very quickly, typically within 30 minutes. They are less accurate than PCR tests, which is why PCR tests are recommended for people with symptoms.

Studies show widely varying estimates of the accuracy of LFTs, typically detecting between 40%-90% of people who have Covid-19 in different studies. We can’t just take an average of these studies to determine the accuracy of the tests as this will depend on a range of factors – such as the type of people in the study, what stage of illness they were at, what specific test was used, and how well the test was carried out.

We do know that carrying out the test correctly is very important. It’s essential therefore to read the instruction leaflet that comes with the test and to watch one of the NHS videos that explain how to carry out a lateral flow test to learn the correct technique.

How do I carry out a lateral flow test?

Before you start, read the instructions that came with the test. If your test needs a throat swab, open your mouth wide and rub the swab over your tonsils (or where they would have been if you have had a tonsillectomy). Put the same swab inside your nose (about 2.5 cm / one inch up or until you feel resistance). If your test requires only a nose swab, put the swab inside your nose (about 2.5 cm / one inch).

After you have done this, put the end of the swab into the tube so it’s in the liquid, then squeeze the liquid from the tube onto the test strip and read the test result once the waiting time advised in the instruction leaflet has passed. Once you have the result, record this via the reporting website. This is important as it allows the government and the NHS to monitor Covid-19 infections in the UK.

Full instructions on how to carry out the test are available on the NHS Website. There are also several NHS videos available online that show people how to use the tests properly.

People online have had negative lateral flows but positive PCRs – why is this? Should I get a PCR instead?

Lateral flow tests and PCR tests work in different ways. Lateral flow tests detect some of the proteins in the virus, can be carried out at home and give a result quickly. PCR tests detect the genetic material (RNA) in the virus and require a laboratory to do this. The result of a PCR test takes longer to come back but it will pick up more infections than a lateral flow test because the test has a higher sensitivity than a lateral flow test.

Because lateral flow tests are not as accurate as PCR tests, they can sometimes be negative in people with symptoms when the subsequent PCR test is positive. If you have symptoms of a possible Covid-19 infection, you should always get a PCR test and not rely on a lateral flow test. If you don’t have symptoms, a lateral flow test is sufficient. PCR tests should be reserved for people with symptoms of a possible Covid-19 infection to avoid overloading government laboratories with unnecessary test requests. (UPDATE: From 11 January, most people with a positive lateral flow test do not need to take a confirmatory PCR test to confirm they have Covid-19).

What are the symptoms of a Covid-19 infection?

When Covid-19 first arrived in the UK in early 2020, the main symptoms people were asked to look out for were a high temperature, a new continuous cough, or a loss or change to their sense of smell or taste. Now that so many people in the UK have some immunity because of vaccination or a previous Covid-19 infection, they may get other symptoms when infected – such as a headache, runny nose, sore throat, or sneezing. So even if you think you might just have a cold, you should still get a PCR test.

How long before meeting family/friends should you take one, and does taking multiple LFTs increase their accuracy?

The test should be done on the day of the meeting. One test is sufficient. Although you can carry out more than one test a day, there is a shortage of tests in many parts of England. If the tests are over-used, this will lead to further supply problems – including for key workers such as NHS staff who are required to test regularly for employment purposes.

Does the thickness of the ‘test’ line on an LFT give any indication of how infectious you are? (if it’s a weak line, does that mean you are hardly shedding any virus? Does a thick one mean you are really infectious?)

The test is designed to tell you if you may be infected and is not designed to estimate how infectious you may be. You shouldn’t therefore use the thickness of the line to try to estimate your infectivity. At a time when the prevalence of Covid-19 in the UK is so high, a positive test means that you are very likely to be infected and you should isolate until you get the result of a PCR test; and then isolate further if this is positive.

If you are still testing positive on an LFT after 10 days’ isolation, are you still infectious and do you still need to isolate? (MIT medical journal says: “But what if someone still tests positive after 10 days? This is very common. People who have tested positive for COVID-19 are very likely to continue to test positive after 10 days. But they are not contagious.”)

There is no need to keep on testing repeatedly if you have had a positive LFT result and this is then confirmed by a PCR test. You should complete your period of isolation and, if in England, carry out any required LFTs as advised by NHS Test and Trace. In England, people who are fully vaccinated can now carry out LFTs on day 6 and day 7 of their isolation period and come out of isolation if both these tests negative. Unvaccinated people will need to continue to isolate for 10 days. Test results can sometimes remain positive for a period after day 10 of isolation ends but this not necessarily mean you are still infectious. (UPDATE: From 17 January 2022, the isolation period in England was reduced to five days if the lateral flow tests are negative on day 5 and day 6).

If you’ve tested positive, including for Omicron, does that mean you won’t get it again?

Reinfections can occur after a previous Covid-19 infection. These are more common with Omicron which a recent study from Imperial College London showed was over 5-times more likely to cause a reinfection than the Delta variant. These reinfections will have been in people who have had an infection with a variant such as Delta, Alpha or the original version of SARS-CoV-2. Because Omicron infections have only occurred recently in the UK, we don’t yet know how prone people who have had an Omicron infection will be to a repeat infection.