What are lateral flow tests?
Lateral flow tests are a type of disposable molecular test used to detect an active infection. No laboratory processing is needed, so they can be used on the spot near the person being tested. They are a cheap technology that can be made in large volumes; this suits mass testing for which large quantities of tests are needed. A nose and throat swab fluid sample is applied to an absorbent pad. The test kit contains antibodies that bind to SARS-CoV-2 proteins on the virus. A positive result is seen as a dark band or a fluorescent glow on the test strip, with a result in about 30 minutes.
Three lateral flow devices have passed the Government’s minimum standards for quality:
- Healgen Coronavirus Antigen Rapid Test.
- SD Biosensor Lateral Flow Test.
- Innova SARS-CoV-2 Antigen test.
Are lateral flow tests different to PCR tests?
Yes. For PCR tests, a swab is used to collect an RNA sample (the nucleic acid that converts DNA into proteins) from the patient's tonsils and inside their nose.
RNA is collected as it carries the genetic information of this specific virus. This is then sent to a laboratory where the sample is heated and cooled so it multiplies into larger quantities of DNA.
Bioscientists can then see whether the SARS-CoV-2 virus (the virus that causes COVID-19) is present.
Because of the process, PCR test results take about two days.
COVID-19 testing is found when using PCR tests to be around 99% effective.
These types of tests have been used by the NHS in their testing centres around the country for the past few months and are the tests sent out by the NHS to people who have symptoms.
How are the accuracy of tests measured?
As with any diagnostic test, data about their accuracy and reliability are crucial and no test can claim 100% accuracy. The most important terms used in describing accuracy are:
- Sensitivity: the proportion of people with SARS-CoV-2 infection who test positive. For example, a test with sensitivity of 95% would mean that 5 in 100 people who have COVID-19 would test negative (false negative). They have an infection, but the test says that they don’t.
- Specificity: the proportion of people without SARS-CoV-2 infection who test negative. For example, a test with specificity of 90% means that 10 in 100 people who are not infected still test positive (false positive). They do not have an infection, but the test says that they do.
Several factors influence overall accuracy when tests are used in real-world settings. This means that manufacturers’ claims of accuracy do not tell the whole story. When levels of infection in the population being tested are high, a test with a high level of sensitivity will be very good at identifying people with an infection but less good at detecting people that do not. Conversely if the level of infection in a population is low, then a higher specificity becomes more important because it will be better at identifying the people that don’t have the infection than it is at detecting the people that do.
How accurate are the UK’s lateral flow tests?
Public Health England’s independent evaluation of how the Innova SARS-CoV-2 Antigen Rapid Qualitative Test worked in a number of community settings reported very high specificity at 99.68%.
This means that for every 1,000 people tested, only 3 people would get a false positive result and subsequently be required to isolate.
However, the tests overall sensitivity was 76.8% meaning that about a quarter of people would not be identified (false negative results).
Sensitivity varied according to who performed the test, showing that performance results not only from a test’s physical features, but how it is used in real life:
- Laboratory scientists: 79.2%
- Healthcare workers: 73%
- Self-trained members of the public following instructions: 57.5%.
What impacts the accuracy of lateral flow tests?
The World Heath Organisation has said that the accuracy of lateral flow tests depends on several factors, including:
- The time from onset of infection
- The concentration of the virus in the specimen
- The quality and processing of the specimen collected from the person.
- The precise formulation of the reagents in the test kits
The quality and processing of the specimen are determined to a large extent by who carries out the tests. By way of example, Public Health England’s evaluation of the Innova test showed that its sensitivity was 79.2% when used by trained laboratory scientists, 73% when used by trained healthcare staff, but only 57.5% when used by track and trace centre staff employed by the pharmacy chain Boots.
The World Health Organisation has also said that lateral flow tests are more likely to detect positive cases when viral loads are highest and patients are most infectious—typically, one to three days before the onset of symptoms and during the first five to seven days after the onset of symptoms.
There is at least a 90% chance that infections will be detected when viral loads are high (over 100,000 copies of viral RNA) but this decreases to less than 60% for lower levels of virus.
WHO’s Essential Diagnostics Test states that negative results should never be used as a basis of decision making. This is why WHO recommends repeat testing using lateral flow devices or preferably confirmatory testing with a PCR test after a negative lateral flow test.