Recent technological developments have introduced the use of flowing systems, which allow chemicals to flow from one environment to another in a continuous fashion. This achieves the “flow state”, and one example of this technology can be found in lateral flow tests. Flow tests aim to detect the presence of a target substance in a liquid sample, abolishing the need for specialized equipment. This approach is currently used for COVID-19 antigen testing and household pregnancy tests.
Types of lateral flow tests (LFTs)
LFTs can be used in a dipstick format or in a housed cassette, both of which work in similar ways. There are two main types of lateral flow assays:
Sandwich assay: A positive test is represented by the presence of a coloured line at the test line position
Competitive assay: A positive test is represented by the absence of a coloured line at the test line position
Read THIS ARTICLE by Abingdon Health for a comparison between different assay types.
How do lateral flow tests work?
LFTs use nitrocellulose membrane immunoassay technology, colored nanoparticles (or labels), and typically antibodies to produce results. When a sample is added, the sample flows along the test device, passing through the conjugate pad into the nitrocellulose membrane and then onto the absorbent pad.
The process by which the sample flows is known as capillary action, where water moves within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. The capillary action in LFTs causes reactive molecules (in Covid-19’s case, the antigen) to flow along the pad and interact with antibodies further up the pad's surface.
So, what are each of the pads for? The sample pad is the first stage of the absorption process, which sometimes contains a filter, to ensure that the sample flow is accurate and controlled. The conjugate pad, which stores the conjugated labels and antibodies, receives the sample. When the target is present, the immobilised conjugated antibodies and labels will bind to the target and continue to migrate along the test. The binding reagents on the nitrocellulose membrane will bind to the target at the test line, as the sample moves along the device. The colored line will form, the density of which depends on the quantity of the target present. Finally, the sample passes through the nitrocellulose membrane onto the absorbent pad, which absorbs the excess sample.
Watch THIS VIDEO by Abingdon Health to see what happens inside a lateral flow test.
Works Cited
Abingdon Health (2017). Two Successful Immunoassay formats in Lateral Flow Devices. [online] Abingdon Health plc. Available at: https://www.abingdonhealth.com/competitive-inhibition-sandwich-immunoassay-formats-lateral-flow/.
Abingdon Health (2019). ANIMATION: How does a lateral flow immunoassay work? [online] Abingdon Health. Available at: https://www.abingdonhealth.com/videos/how-does-a-lateral-flow-immunoassay-work/.
Abingdon Health (2022). What is a lateral flow test and how does the technology work? [online] Abingdon Health plc. Available at: https://www.abingdonhealth.com/services/what-is-lateral-flow-immunoassay/.
Maynooth University (n.d.). Science Week: How Covid lateral flow tests work | Maynooth University. [online] www.maynoothuniversity.ie. Available at: https://www.maynoothuniversity.ie/research/spotlight-research/science-week-how-covid-lateral-flow-tests-work.
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