As the coronavirus pandemic continues to rage across the world, diagnostic tests have been some of the most powerful weapons against the virus spread. There has been a slew of different tests thrown into the mix, ranging from accurate but laborious PCR tests to quick and easy home antigen tests.
One such test that has risen to the forefront is the lateral flow test. If you are British, you are likely already familiar with these – the UK government is currently offering free lateral flow tests to anyone that wishes to take them. Lateral flow tests are now the "gold standard" for rapid, take-at-home tests for COVID-19, but what are they, and how do they work?
What is a lateral flow test?
Lateral flow tests, more technically known as lateral flow assays, are a single strip diagnostic test that allows for rapid and easy testing for a target substance, whether that is its presence or absence. Most people have been exposed to lateral flow tests way before COVID-19 – they are used widely as an immediate pregnancy test and can be bought in most supermarkets.
These tests shine by being incredibly easy and cheap to make, and they are able to deliver results within minutes.
A standard lateral flow test will involve a small window that displays a strip with two or more marked regions (usually one test region and one control region), and an area at the end where the sample is placed. Once a droplet is placed in the input, it will slowly reveal a band in the test area if positive, or stay blank if negative. The control region must show a band for the result to be considered valid, regardless of the outcome.
Due to their rapid test time, generally accurate results, and established production system, lateral flow tests have now been deployed as a means of tracing the spread of COVID-19. They are taken at home and can detect the presence of the COVID-19 virus, SARS-CoV-2, within the system of a person that provides a swab from the nose and/or throat.
How does a lateral flow test work?
A virus particle is made up of a variety of proteins, whether that be proteins that create the structure of the virus, or toxins they release into the host system. In a SARS-CoV-2 virus, there are four main protein groups – spike, envelope, membrane, and nucleocapsid proteins. You will likely have heard of spike (or ‘S’) proteins already – these are the protrusions that allow the virus entry into host cells. If the protein can bring about an immune response, scientists call it an antigen. Antigens are often specific, so if you can detect the antigen of a virus within a host, the right test will tell you exactly what it is.
For SARS-CoV-2, lateral flow tests detect the virus antigen in a sample taken from the host nose or throat. These tests use a "sandwich" construction, in which if there are antigens present in the sample, it will provide a band with an intensity proportional to the amount present.
First, a liquid sample is placed at the end of the test. The sample is absorbed into a pad, where it moves up into a region filled with fluorescent-labeled antibodies. If there is antigen present, the antibodies bind to the antigen and get carried up the pad in the sample. As these antibodies flow over the test lines, proteins attached to the pad surface bind to the labeled antibodies, generating a signal, which appears as the line seen in the window.
Are lateral flow tests effective?
Recent studies have suggested that lateral flow tests, when performed correctly, are extremely accurate at identifying COVID-19 in symptomatic patients. In a cohort of 2,562 patients that received a lateral flow test and a PCR test, the sensitivity (how often a test is correct at identifying positive results) of lateral flow tests were 95.4%, while the specificity (how often a test is correct at identifying true negative results) was 89.1%. This is true when the test is taken at the start of COVID-19 symptoms, but results may vary if the test is taken at a later time.
Results suggest that lateral flow tests are therefore a strong alternative to PCR tests, at least for those with symptoms, which refutes the idea that they are less sensitive than the more laborious and expensive PCR method. However, as lateral flow tests are commonly taken by the individual at home, it does leave open the potential of human error when performing the test. This is something that is recognized as a confounding factor in the UK by offering a follow-up PCR test.