Lateral Flow Assays: Principles, Designs, and Labels
Lateral Flow Assays are the devices that are used to conduct different types of testing and point of care testing and it is also known as realistic testing. Lateral Flow Assays are small and minimal devices that are being used in almost every field of the medical where the detection of analytes is present. Looking at the current scenario globally, they are also being used for testing Coronavirus. As they are reliable, cost-effective, and accessible to everyone it is used for pregnancy tests, bacterial infections, viral infections, and parasitic infections. Lateral flow Assays tests do not require any kind of trained or specialized personnel. The lateral flow assay technique works upon the biochemical amalgamation of the antibodies and antigens or DNA target- DNA hybridization.
Principles
The principles on which the Lateral Flow Assay works are quite basic. In the Lateral Flow Assay, the liquid sample or the matrix flows with the help of the eternal forces i.e. capillary action. The matric has the target analyte which is necessary for the detection ad production of results. The liquid flows through the various polymeric strips where the analytes and molecules can be combined. The whole principle works on the different parts of the Lateral Flow assay as there are different pads through which the liquid has to flow for producing the results. The first step to use this device is that the sample is put on the device that is flowed to the lateral flow device fully. Once it flows the filtration process occurs where the unwanted or undesirable particulates are neutralized or filtered. Within the given duration of the Lateral flow assay test completion, the whole process occurs and thus results can be seen. When the sample reaches the conjugate pad of the device the dried particles on the pad get intermixed with the particles of the sample. The next step that happens is the flow of analyte-bound nanoparticles and getting access to the control lines that further leads to the results. The specimen can then start flowing to the conjugate pad, which contains highly colored or fluorescent nanoparticles with an antibody on their top layer. There is also a wicking pad that absorbs the sample or specimen liquor completely to ensure that there is no liquid left behind around the test and control lines. After the process completes by following its principle of working the results are achieved.
Designs
The designs of lateral flow assay are composed of a chromatographic system. In the chromatography system, the components are separated based on their movement through the reaction membrane and also the immune reaction as of the antigen and antibody reaction or the DNA hybridization, and nucleic-acid target analyte. Moreover, lateral flow Assay is designed with a sample pad, conjugate bad, nitrocellulose membrane, labels, nanoparticles, adsorbent wicking pad, and the buffer attached to it in some devices only. The designs of lateral flow assays are dependent on the usage, stability, performance, and safety. There are modules that must be followed for the formation of the lateral flow assays and test devices. In the lateral flow assay designs, the physical requirements are very important. These physical requirements of the LFA are dimensions of the cartridge, features of cartridge i.e. conjugate pad and sample pad, readers specifications, etc. The design of LFA is also dependent on the functionality and characteristics of a particular design. For example, the functionality and characteristics related to it will include the method of analysis, format for the test target analyte, the clinical performance, sensitivity and specificity limit, and so on. The two other important factors in the design of the lateral flow assay are handling and sapling requirements. Handling requirements of the design of LFA are more pertinent to the environment, and the shell life. Whereas the sampling requirements of the design for the lateral flow assay test are the types of sample, the temperature at which the sample can be stored, the temperature required for storage, and the treatment for the sample.
Labels
In Lateral Flow Assay, there are different materials used which are given the name of labels. Some of the labels that are used in LFA are gold nanoparticles, quantum dots, colored latex beads, silver nanoparticles, selenium nanoparticles, enzymes, liposomes, and many others. Here are a few of the labels in LFA that will be discussed. Any label being used in the Lateral Flow Assay must be detectable at a very low concentration and the properties of labels should be held till the conjugation with the biorecognition molecules. Firstly, colloidal gold nanoparticles are one of the most common labels. Colloidal gold particles are immobilized and have the perfect sphere shape. Moreover, colloidal gold nanoparticles have a relatively high affinity towards the biomolecules which makes their functionality easy. Their size and shape vary. Some of the most prominent characteristics of the gold nanoparticles are their environment-friendly composition, their high affinity towards the molecules and proteins, good optical signaling, and their stability. Another label that is used in LFAs is magnetic particles and aggregates that are used for signaling in the devices. Magnetic colored particles in LFA are used or producing colored lines that are read by the instrument or a strip reader. There is also a magnetic assay reader that is used to read the signals coming from them. Enzymes are also used in LFA and they increase one step in the whole process. It adds the step of adding the suitable substrate after the assay has been completed. Colloidal carbon is also used in LFAs and black color. Because of their color, they are easily detectable with high sensitivity.
Conclusion
The Lateral Flow Assay tests are one of the most used devices. The principles, designs, and labels of the lateral flow assay almighty be different in some of the Lateral Flow Assay devices that also modify the working of that particular lateral flow assay device. All of the principles, designs and labels are important parts of the LFAs.
References
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4986465/
https://nanocomposix.com/pages/lateral-flow-assay-design
https://www.nature.com/articles/s41598-018-35694-9
https://www.sciencedirect.com/science/article/pii/S131961031400129X