Lateral flow tests (also called lateral flow assay, lateral flow immunoassay, lateral flow device, lateral flow immunochromatographic assay, or rapid tests) are diagnostic devices that help detect diseases, conditions, or contaminations. This is done by being able to detect the presence of analytes which can be pollutants or pathogens.
Lateral flow tests are popular because they are reliable, easy to administer, and are very affordable as they don’t need specialized equipment to be able to bring results. Lateral flow tests can be done in the laboratory, in a clinic, and even at home, so one can simply buy lateral flow test kit for a certain condition and do it at home. In fact, pregnancy test kits are some of the most popular examples of lateral flow tests.
Recently though, rapid flow tests are most known for its role in easily detecting COVID-19 in symptomatic patients. Rapid test kits for COVID-19 are actually rapid flow tests where mucus of a patient is taken using a throat swab or a nose swab, diluted by a solution, then placed on the test kit, showing whether or not the SARS-CoV-2 virus is present.
But aside from pregnancy tests and rapid COVID-19 tests, there are a lot more kinds of lateral flow tests, and they are typically categorized as follows:
Types of Lateral Flow Test according to Assay Formats
There are two types of lateral flow tests depending on their assay format.
The first one is sandwich assay format, wherein the nanoparticle in the test line usually contains the antibody. Such antibodies will bind to the sample if the target analyte is present. This will come out to the test as a colored line (or label) in the test line, indicating a positive result. The test line will actually look intensified depending on the amount of the analyte present. Thus, a positive result in the sandwich assay format is a colored line in the test line.
On the other hand, a competitive assay works differently. The antibody will be in the conjugate pad instead of the test line. If the analyte from the sample is already bonded with the conjugate, it will no longer bind with the test line. Therefore, a positive result in the competitive assay is the absence of a colored line in the test line.
Types of Lateral Flow Test according to Nanoparticle Reporters
Aside from assay formats, there are also types of lateral flow tests depending on the reporter nanoparticle used. These nanoparticles are also called labels.
Gold nanoparticles are some of the most common, and they are used because of their strong light absorption property. Other common labels are carbon and colored latex. Magnetic beads and colored polystyrene beads are also used as labels. The choice of label depends on a number of factors including the target analyte, the matrix, and the antibody. Their optimal interaction will help ensure accuracy of results.
Types of Lateral Flow Test according to Assay Analysis
Finally, there are also different types of lateral flow tests depending on how the assay is analyzed. Qualitative assays simply yield a positive or negative result depending on the presence of the analyte.
Meanwhile, quantitative assays test for the intensity of the test line. This can be done by comparing the results to a calibration standard, or running through a device or reader. There are lateral flow tests where determining the intensity of the test line (indicating the amount of analyte present) is crucial.
The type of lateral flow test that you’ll encounter really depends on what experts see fit to be able to detect and diagnose as accurately as possible. Developments in lateral flow tests will continue to provide easy, affordable, and reliable ways to detect diseases so we can treat them accordingly.