Lateral flow tests[1] also known as Lateral Flow Immunochromatographic Assays are a simple device intended to detect the presence (or absence) of a target analyte in sample (matrix). Most commonly these tests are used for medical diagnostics either for home testing, point of care testing, or laboratory use. Often produced in a dipstick format, Lateral flow tests are a form of immunoassay in which the test sample flows along a solid substrate via capillary action. After the sample is applied to the test it encounters a coloured reagent which mixes with the sample and transits the substrate encountering lines or zones which have been pretreated with an antibody or antigen. Depending upon the analytes present in the sample the coloured reagent can become bound at the test line or zone. Lateral Flow Tests can operate as either competitive or sandwich assays.
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In principle any coloured particle can be used, however commonly either latex (blue colour) or nanometer sized particles of gold (red colour) are used. The gold particles are red in colour due to localised surface plasmon resonance. Fluorescent[2] or magnetic[3][4] labelled particles can also be used - however these require the use of an electronic reader to assess the test result.
The sample first encounters coloured particles which are labelled with antibodies raised to the target analyte. The test line will also contain antibodies to the same target, although it may bind to a different epitope on the analyte.
The test line will show as a coloured band in positive samples.
See also, for example, sandwich ELISA.
The sample first encounters coloured particles which are labelled with the target analyte or an analogue. The test line contains antibodies to the target/its analogue. Unlabelled analyte in the sample will block the binding sites on the antibodies preventing uptake of the coloured particles.
The test line will show as a coloured band in negative samples.
Quantitative tests == Most tests are intended to operate on a purely qualitative basis. However it is possible to measure the intensity of the test line to determine the quantity of analyte in the sample. Handheld diagnostic devices known as lateral flow readers are used by several companies to provide a fully quantitative assay result. By utilizing unique wavelengths of light for illumination in conjunction with either CMOS or CCD detection technology, a signal rich image can be produced of the actual test lines. Using image processing algorithms specifically designed for a particular test type and medium, line intensities can then be correlated with analyte concentrations. One such handheld lateral flow device platform is made by Detekt Biomedical L.L.C., [1]. Alternative non-optical techniques are also able to report quantitative assays results. One such example is a Magnetic immunoassay (MIA) in the lateral flow test form also allows for getting a quantified result.
While not strictly necessary, most tests will incorporate a second line which contains an antibody that picks up free latex/gold in order to confirm the test has operated correctly.
Time to obtain the test result is a key driver for these products. Tests can take as little as a few minutes to develop. Generally there is a trade off between time and sensitivity - so more sensitive tests may take longer to develop. The other key advantage of this format of test compared to other immunoassays is the simplicity of the test - typically requiring little or no sample or reagent preparation...
Probably the most well known examples of lateral flow tests are home pregnancy tests. However rapid tests or point of care tests are available for a wide range of applications including: HIV tests, Troponin T, test Malaria tests, drugs of Abuse tests, Fertility tests, Respiratory disease tests etc.
Clinical tests can be applied to urine, saliva, blood, or stool samples. Tests are available for both human and animal diagnostics. Tests are also available for non clinical applications including testing food and water[5] for contaminants as well as biothreat agents and other environmental contaminants. [6]
This is a highly competitive area and a number of people claim patents in the field, most notably Inverness Medical Innovations who own patents[7] originally filed by Unipath. A group of competitors to Inverness Medical Innvoations are challenging the validity of the patents.[8] A number of other companies also hold patents in this arena.
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Manufacturing technology of Lateral flow assay kits [2]
Developing highly sensitive, more-reproducible lateral-flow assays Part 1: New approaches to old problems
Developing highly sensitive, more-reproducible lateral-flow assays Part 2: New challenges with new approaches [4]
Lateral-flow assays: Designing for automation [5]
Lateral-flow POC tests to grow [6]
Effects of adhesive migration in lateral-flow assays [7]
Biochemicals and chemical reagents [8]
Hi-Flow Plus Membranes for lateral flow [9]
Diagnostic Membranes for Lateral Flow Assays [10]
Lateral Flow Diagnostics [11]
Introduction to Lateral Flow [12]
Reader for colorimetric or fluorescence lateral flow tests [13]
Handheld lateral flow strip reader developer [14]
Lateral Flow assay In Animal Disease Diagnosis [15]