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Rapid lateral flow testing

Laboratory based polymerase chain reaction (PCR) tests are seen as the “gold standard” test for identifying clinical cases or those with symptoms of Covid-19 infection. But they take time, are relatively expensive and must be processed in a laboratory.

COVID-19 lateral flow tests are a way of quickly testing for SARS-CoV-2 in asymptomatic people. Asymptomatic people (estimated to be 1 in 3 people with Covid-19) have the virus and can spread it unknowingly. By doing the lateral flow testing on asymptomatic people every three to four days we can more easily identify those with Covid-19 and take precautions to prevent further spread.

The test kits are small and portable, making them easy to use outside of the healthcare setting. The result is usually available within 30 minutes (compared to the 24 hours of PCR testing), making them an ideal choice when testing large numbers of workers, for example.

How do lateral flow tests work?

When doing a lateral flow test for Covid-19 a swab is swept across the back of a person’s nose and throat. The swab is then placed into a tube of liquid, after which a sample of this liquid is deposited on a small absorbent pad contained within the disposable testing kit. Information on how to prepare for and perform the test is available here. The liquid is drawn along the pad by capillary action, until it encounters a strip coated in antibodies which are specific to the SARS-CoV-2 virus antigen. If antigens are present, this will show up as a coloured line – indicating a positive test.

Who should have a lateral flow test?

The lateral flow tests are particularly useful for people who cannot work from home, including health workers, care home staff, teaching staff, students allowed to attend higher education institutions and employees of companies that have signed up to a government testing scheme. In the UK, for example, the testing kits are made freely available to most people and the government encourages testing. Self-tests are generally conducted twice a week before users leave home.

How accurate are they?

The speed and convenience of these tests comes at the expense of accuracy, meaning care needs to be taken with the interpretation and communication of results. When it comes to reporting the accuracy or validity of diagnostic tests, there are two key considerations: sensitivity and specificity.

Explanation of sensitivity and specificity

  Sensitivity Specificity
Explanation
  • refers to the proportion of true positives a test correctly identifies
  • the higher the sensitivity the lower the number of false positive cases (people reported to have the disease who do not have it)
  • refers to the percentage of true negatives a test correctly identifies
  • the higher the specificity the lower the number of false negative tests (people told they do not have the disease when they do)
Example 97% sensitivity means 3 in 100 people will be told they have Covid-19 when they do not 80% specificity means that 20 in every 100 people would be told they do not have Covid-19 when they do

 

The relevance of this is that if a test has low specificity, then many more positive Covid-19 cases are misdiagnosed, allowing individuals to falsely think they do not have Covid-19. This results in their not quarantining and therefore being left to continue to spread the virus in the community.

The World Health Organization recommends that rapid diagnostic tests for SARS-CoV-2 must have a minimum sensitivity of 70% and at least 97% specificity. However, research has shown that the quality and processing of the specimen may also be determined by those who carry out the tests. In one study, the sensitivity of a test was highest when used by trained laboratory staff, followed by trained healthcare staff, and relatively low when used by test and trace centre staff working for a large pharmacy chain. Even though they are less accurate than PCR tests, their low cost, speed and ease of use makes lateral flow tests particularly attractive when screening large numbers of people with the need for a quick turnaround time.

lateral flow tests will help to identify people who are infected but asymptomatic, getting them to quarantine and so reduce transmission of the virus.

Additional resources

The US CDC provides detailed information on how to test for current and past testing  

The NHS provides advice and guidance on the use of lateral flow testing on the following websites: