Lateral flow tests, or antigen tests, are the rapid tests which tell people whether or not they have coronavirus.
Some pharmacies also placed signs inside their branches saying they were temporarily out of stock.
Meanwhile, health experts have been discussing when is the appropriate time to take the tests as people can change from being testing positive to negative in a matter of hours.
The PA news agency answers some key questions about lateral flow tests.
– What is the difference between lateral flow tests and PCR tests?
Lateral flow tests provide results and tell people in 30 minutes if they have coronavirus by detecting proteins from the virus in the nose and throat samples. Scientists have mixed views on their accuracy.
PCR tests are, however, extremely accurate but take up to three days for results to come through. They detect the genetic material from a specific organism, specifically coronavirus, and are the best way to test if you have a current infection.
Both tests require swabs from your nose and throat.
– When should you take lateral flow tests?
According to the Royal College of Pathologists, lateral flow tests should be taken by people who do not have symptoms.
A factsheet on the RCP’s website states: “These tests are very different from PCR. They are not suitable for diagnosing individual patients who suspect they may be infected because they have symptoms.
“People with symptoms need a PCR test. Lateral flow tests are intended for picking up additional infected cases who would otherwise be missed because they don’t have any symptoms.”
On the NHS website, it also says that people who have symptoms of coronavirus should also complete a lateral flow test rather than a PCR test.
Current advice states that if you test positive on a lateral flow you should follow up with a PCR test.
People are advised to do lateral flow tests before mixing with crowds in indoor places or visiting someone who is at high risk of getting Covid-19.
It’s also advised that if you’re vaccinated, but have been in contact with someone who then tests positive for coronavirus, you should do a lateral flow test.
On Sunday, Irene Petersen, a professor of epidemiology at University College London, said official advice should be updated to say that people should take the tests just before they are about to meet others due to the fast infection rate of Omicron.
She added that tests results “expire quickly” as people “may switch from being non-infectious to infectious within hours”.
Government guidance currently recommends taking a test “if you will be in a high-risk situation that day”.
Professor Stephen Reicher told BBC Breakfast on Monday that people should be doing lateral flow tests before meeting up with friends and family this Christmas.
– How accurate are they?
A study by the Queen Mary University of London, the University of Oxford, the Institute for Advanced Studies based in Vienna, and the Medical University of Graz published in July found that lateral flow tests detected more than 95% of the cases found by PCR and correctly identified 89% of cases as negative.
In March 2021, the Royal College of Pathologists said positive results by LFTs should be confirmed by PCR tests and people should self-isolate before they receive their PCR result.
Last week, the Health Security Agency (HSA) said lateral flow tests are as likely to detect Omicron as other variants of coronavirus.
PCR tests are still considered to be the most accurate tests when diagnosing coronavirus but should only be used by people who are showing symptoms.
– How can you get hold of lateral flow tests?
The NHS has a searching tool which allows people to find their nearest pharmacies which have tests available to collect. It is: www.maps.test-and-trace.nhs.uk.
Alternatively, people can order a pack from Gov.uk to be delivered to their homes. One pack can be ordered per day.
— UK Health Security Agency (@UKHSA) December 14, 2021
They can also be collected from community collection points or people can visit a testing point near to their homes.
– Is there still an issue with getting hold of lateral flow tests?
Last week, people reported being unable to order lateral flow tests on the NHS website while pharmacies displayed posters saying they had run out of stock.
People trying to book walk-in PCR tests also encountered problems, with some being directed to sites several miles away or being told no slots were available.
Dr Jenny Harries chief executive of the UK Health Security Agency (UKHSA), downplayed suggestions of a shortage, and said requests for lateral flow tests “have been absolutely astounding”, with “unprecedented demand” for PCR tests.
Sir Chris Wormald, permanent secretary at the Department of Health and Social Care, said issues getting hold of tests laid with distribution, rather than availability.
The Company Chemists’ Association (CCA), the trade association for large pharmacy operators in England, Scotland and Wales including Rowlands and Superdrug, said demand had outstripped supply in some areas.
Chief executive Malcolm Harrison said: “CCA members continue to receive supplies of lateral flow tests but we are aware that in a few areas there have been difficulties as demand outstrips supply, especially in the lead up to Christmas.
“Pharmacies are managing the demand as best as they can and we urge members of the public to be patient at this challenging time for us all.”
A spokesperson from Boots said the chain provides lateral flow tests at more than 2,100 pharmacies in the UK but “there may be a small number of stores that are experiencing temporary shortages due to higher demand in certain areas”.
Lloyd’s Pharmacy said tests were available but there was “significant demand”.
Currently, there are no issues with ordering tests on the Government’s website.