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.
At the peak of the COVID-19 pandemic, lateral flow tests were a way of quickly screening for SARS-CoV-2 in asymptomatic people. By doing the lateral flow testing on asymptomatic people we could more easily identify those with Covid-19 and take precautions to prevent further spread. Due to the success of the vaccination campaign the screening of asymptomatic people and symptomatic persons is no longer required. As of 11 May 2022 in the UK free PCR or lateral flow testing is no longer available to the general public. Free testing is still available to high-risk groups.
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?
To protect the most vulnerable, symptomatic testing in high-risk settings remains important to ensure that COVID-19 is detected as quickly as possible. During periods of high prevalence of Covid-19 asymptomatic testing will continue in these high-risk settings. Members of the general public or workers not employed in high-risk settings will have to pay for the test. The cost of any testing required by an employer should be covered by the employer requesting the test.
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
|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.
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 website: