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The lateral flow test (LFT) is an assay also known as a lateral flow device (LFD), lateral flow immunochromatographic assay, or rapid test. It is a simple device designed to detect the presence of target substances in liquid samples without the need for specialized and expensive equipment.LFTs are widely used for medical diagnostics in the home, point-of-care, and laboratory.For example, a home pregnancy test is an LFT that detects specific hormones.These tests are simple and affordable, and usually show results in about five to thirty minutes.Many laboratory-based applications increase the sensitivity of simple LFTs by using additional specialized equipment.Since the target substance is usually a biological antigen, many lateral flow tests are rapid antigen tests (RAT or ART).LFT operates on the same principle as affinity chromatography in an enzyme-linked immunosorbent assay (ELISA). Essentially, these tests run a liquid sample along the surface of the pad with reactive molecules, showing a visually positive or negative result.These pads are based on a series of capillary beds such as porous paper sheets microstructured polymers or sintered polymers.Each of these pads has the ability to spontaneously transport fluids (e.g., urine, blood, saliva).The sample pad acts as a sponge and holds excess sample fluid.After soaking, the liquid flows to a second conjugate pad, where the manufacturer stores freeze-dried bioactive particles called conjugates (see below) in a salt-sugar matrix.The conjugate pad contains all the reagents needed to optimize the chemical reaction between the target molecule (e.g. antigen) and its chemical partner immobilized on the particle surface (e.g. antibody).This marks the particles of interest as they pass through the pad and continue across the test and control lines.The test line shows a signal, usually the color in a pregnancy test.The control line contains the affinity ligand and shows whether the sample is flowing through and the biomolecules in the conjugated pad are active.After passing through these reaction zones, the fluid enters the final porous material, the wick, which simply acts as a waste container.LFT can be performed as a competition or sandwich assay.
History 
LFT was derived from paper chromatography, which was developed by Martin and Synge in 1943 and elaborated by Consden, Gordon and Martin in 1944.After 1945, activity in this field exploded.The ELISA technique was developed in 1971.Beginning in 1988, Armkel LLC filed a group of LFT patents, including the suit described below US 6,485,982
Synopsis
Colored particles
In principle, any colored particles can be used, but latex (blue) or nano-sized gold particles (red) are most commonly used.Gold particles appear red due to localized surface plasmon resonance.Fluorescent or magnetic labeled particles can also be used, but these require the use of an electronic reader to evaluate test results.
Sandwich test
Sandwich methods are typically used for larger analytes because they tend to have multiple binding sites.As the sample migrates through the assay, it first encounters a conjugate, an antibody specific for the analyte of interest labeled with a visual label, usually colloidal gold.Antibodies bind and migrate with target analytes in the sample until they reach the test line.The test line also contains immobilized antibodies specific to the analyte of interest that bind to the migrating analyte-bound conjugated molecules.Due to the concentrated visual label, the test line then exhibits a visual change, confirming the presence of the target molecule.Most sandwich assays also have a control line that is present regardless of the presence or absence of the analyte of interest to ensure proper function of the lateral flow pad.This rapid, low-cost sandwich-based test is commonly used in home pregnancy tests to detect human chorionic gonadotropin (hCG) in the urine of pregnant women.
Competitive assays
Competitive assays are typically used for smaller analytes because smaller analytes have fewer binding sites.The sample first encounters antibodies directed against target analytes labeled with visual labels (colored particles).The test line contains the analyte of interest immobilized on the surface.When the analyte of interest is not present in the sample, unbound antibodies will bind to these immobilized analyte molecules, which means a visual marker will be displayed.Conversely, when the target analyte is present in the sample, it binds to the antibodies to prevent them from binding to the immobilized analyte in the test line, so no visual markers are shown. This differs from sandwich analysis as the absence of a band means the analyte is present.
Quantitative tests
Most LFTs are designed to operate on a purely qualitative basis.However, the strength of the test line can be measured to determine the amount of analyte in the sample.Some companies use handheld diagnostic devices called lateral flow readers to provide fully quantitative assay results.By illuminating with a unique wavelength of light combined with CMOS or CCD detection technology, signal-rich images of the actual test line can be generated.Using image processing algorithms designed for specific test types and media, line intensities can then be correlated to analyte concentrations. Detekt Biomedical L.L.C.manufactures one such handheld lateral flow device platform.Alternative non-optical techniques are also capable of reporting quantitative results.One such example is the LFT format of magnetic immunoassay (MIA) which also allows quantitative results to be obtained.Reducing variation in capillary pumping of sample fluids is another way to move from qualitative to quantitative results.For example, recent work has demonstrated that capillary pumping has a constant flow rate independent of liquid viscosity and surface energy.
