Lateral flow tests 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

Immunoassay

An immunoassay is a biochemical test that measures the presence or concentration of a substance in solutions that frequently contain a complex mixture of substances. Analytes in biological liquids such as serum or urine are frequently assayed using immunoassay methods...

 in which the test sample flows along a solid substrate via capillary action

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

. 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.

Coloured particles

In principle any coloured particle can be used, however commonly either latex (blue colour) or nanometer sized particles of gold

Colloidal gold

Colloidal gold is a suspension of sub-micrometre-sized particles of gold in a fluid — usually water. The liquid is usually either an intense red colour , or a dirty yellowish colour ....

 (red colour) are used. The gold particles are red in colour due to localised surface plasmon resonance

Surface plasmon resonance

The excitation of surface plasmons by light is denoted as a surface plasmon resonance for planar surfaces or localized surface plasmon resonance for nanometer-sized metallic structures....

. Fluorescent or magnetic
labelled particles can also be used - however these require the use of an electronic reader to assess the test result.

Sandwich assays

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

Epitope

An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope...

 on the analyte.

The test line will show as a coloured band in positive samples.

See also, for example, sandwich ELISA.

Competitive assays

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., http://www.idetekt.com. Alternative non-optical techniques are also able to report quantitative assays results. One such example is a Magnetic immunoassay

Magnetic immunoassay

Magnetic immunoassay is a novel type of diagnostic immunoassay using magnetic beads as labels in lieu of conventional enzymes , radioisotopes or fluorescent moieties . This assay involves the specific binding of an antibody to its antigen, where a magnetic label is conjugated to one element of...

 (MIA) in the lateral flow test form also allows for getting a quantified result.
Control line

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.

Speed & Simplicity
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...
Examples
Probably the most well known examples of lateral flow tests are home pregnancy tests. However rapid tests or point of care tests

Point-of-care testing

Point-of-care testing is defined as medical testing at or near the site of patient care. The driving notion behind POCT is to bring the test conveniently and immediately to the patient. This increases the likelihood that the patient, physician, and care team will receive the results quicker,...

 are available for a wide range of applications including: HIV test

HIV test

HIV tests are used to detect the presence of the human immunodeficiency virus , the virus that causes acquired immunodeficiency syndrome , in serum, saliva, or urine. Such tests may detect antibodies, antigens, or RNA.- Terminology :...

s, Troponin T

Troponin T

Troponin T is a part of the troponin complex. It binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex.The tissue specific subtypes are:* Slow skeletal troponin T1, TNNT1 * Cardiac troponin T2, TNNT2...

, test Malaria

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 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 for contaminants as well as biothreat agents and other environmental contaminants.

Patents
This is a highly competitive area and a number of people claim patents in the field, most notably Inverness Medical Innovations who own patents originally filed by Unipath

Unipath

Unipath Ltd is a large pharmaceutical company specialising mainly in medical diagnostic kits related to women's reproductive health. Recently, it has been diversifying its product range to include more specialised test kits, primarily for the clinical market, in areas outside of women's...

. A group of competitors to Inverness Medical Innvoations are challenging the validity of the patents. A number of other companies also hold patents in this arena.
External links
Manufacturing technology of Lateral flow assay kits

Developing highly sensitive, more-reproducible lateral-flow assays
Part 1: New approaches to old problems

• http://www.devicelink.com/ivdt/archive/06/06/010.html

Developing highly sensitive, more-reproducible lateral-flow assays
Part 2: New challenges with new approaches http://www.devicelink.com/ivdt/archive/06/07/011.html

Lateral-flow assays: Designing for automation http://www.devicelink.com/ivdt/archive/99/05/001.html

Lateral-flow POC tests to grow http://www.devicelink.com/ivdt/archive/07/05/005.html

Effects of adhesive migration in lateral-flow assays http://www.devicelink.com/ivdt/archive/00/09/006.html

Biochemicals and chemical reagents http://www.devicelink.com/ivdt/archive/04/10/002.html

Hi-Flow Plus Membranes for lateral flow http://www.millipore.com/techpublications/tech1/PF10019EN00

Diagnostic Membranes for Lateral Flow Assays http://www.whatman.com/DiagnosticMembranesforLateralFlowAssays.aspx

Lateral Flow Diagnostics http://www.millipore.com/diagnostics/dp3/lateral

Introduction to Lateral Flow http://www.biodot.com/applications/lateral_flow/lf_main_page.htm

Reader for colorimetric or fluorescence lateral flow tests http://www.ese-gmbh.de/logicio/pmws/indexDOM.php?client_id=ese&page_id=lfr&lang_iso639=en

Handheld lateral flow strip reader developer http://www.idetekt.com

Lateral Flow assay In Animal Disease Diagnosis http://www.slideshare.net/bishorvi/lateral-flow-assay-in-animal-disese-diagnosis