EXPLAINER: What is COVID lateral flow, UK’s replacement for PCR arrival test?

On Wednesday, the United Kingdom said from Friday, it will no longer require vaccinated travellers to take COVID tests before boarding flights to the country.

According to Boris Johnson, UK prime minister, the requirement for travellers to self-isolate on arrival until receipt of a negative polymerase chain reaction (PCR) test would also be scrapped from Sunday.

Instead, new arrivals will be required to take a lateral flow test no later than day two.

If a traveller tests positive with a lateral flow, they will then be required to take a PCR test and self-isolate for up to 10 days.

WHAT IS A LATERAL FLOW TEST?

A lateral flow test is a quick test for COVID that gives result in 30 minutes. It is recommended for people who do not have symptoms of the virus.

The test kit is a hand-held device with an absorbent pad at one end and a reading window at the other.

A sample of mucus is taken from a person’s nose or throat using a swab. The swab is then dipped in a tube containing a solution to dilute the sample, and then dripped on to the device’s paper pad. A coloured band will appear on the control zone labelled C, to show the test has worked.

If a coloured band also appears on the test zone labelled T, it indicates a positive test.

Lateral flow tests were already used widely even before the COVID pandemic. Their most common use is for pregnancy — they detect a hormone present in pregnant women’s urine.

HOW IS IT DIFFERENT FROM A PCR TEST?

PCR tests work by detecting the virus’ genetic material, while lateral flow tests work by detecting proteins from the virus, which are present when someone is infectious.

Also, although both tests require one to take a swab from the nose or throat, PCR tests have to be sent to a laboratory while lateral flow tests can easily be done at home and give results in minutes.

WHY IS IT RECOMMENDED FOR PEOPLE WITHOUT SYMPTOMS?

According to the NHS, about one in three people with COVID do not have symptoms but can still infect others. The rapid lateral flow tests help to find positive cases earlier and to break hidden chains of transmission.

HOW EFFECTIVE IS THE LATERAL FLOW TEST FOR COVID?

Research shows that rapid tests are reliable for detecting the coronavirus. A recent study showed that lateral flow tests, when used correctly, are likely to have a sensitivity above 80% and in many cases above 90%.

Analysis by NHS ‘Test and Trace’ also showed that the flow tests have an estimated specificity of at least 99.97% when used in the community.

CAN IT DETECT THE OMICRON VARIANT?

Although the Omicron variant is reported to have over 30 mutations, the lateral flow tests can still detect a COVID infection caused by the Omicron variant.

According to the UK Health Security Agency, “preliminary laboratory data assess these devices as detecting Omicron. Initial data from contrived samples indicate a comparable sensitivity to that observed for previous strains of SARS-CoV-2 including Delta”.