The most important immunological rapid tests are so-called strip tests in a lateral flow assay format in which a porous membrane represents the assay carrier. Hundreds of millions of lateral flow assays (LFA) are currently sold worldwide for medical diagnostics. Due to low production costs and an easy handling the lateral flow test strip is an ideal platform for point-of-care applications. In many cases, a single step of sample application is sufficient

 

Operation

The ability of the human body to detect foreign objects is taken advantage of in the development of rapid tests. The principle is that foreign substances such as proteins, called antigen, are detected by the body's own proteins, called antibodies, by connecting both substances together. Thus, an immune response is triggered that ensures that phagocytes track down and dispose the invading antigen. In the diagnostics and research monoclonal antibodies play a major role, since they can bind a variety of molecules with high specificity: they only bind to a specific epitope (binding site) of the antigen, which can be detected very well. The body's immune system, however, is always aligned polyclonal. It can bind to different epitopes of an invading antigen. 

By labeling antigens with specific color molecules or enzymes they are made visible on a membrane, when they occur more frequently at one location.

  

Construction of the test strip and test procedure

The lateral flow assay demonstrates the presence of an analyte by visual interpretation of color development on a test strip.

Immunological LFAs are based on the same principle as other immunological tests: the detection of an analyte in a liquid phase by binding to a recognition structure, such as an antibody. The detection structure (antibody) is applied on a nitrocellulose (NC) membrane and forms the basis for the specific band signal on the NC membrane. Next to the NC Membrane, an LFA consists of three other different membranes that exist overlapping on a carrier (backing card): First is the sample pad, used for sample loading. From there, the sample flows into the conjugate pad, where antibody conjugates are released. In general, antibody conjugates with gold nanoparticles or latex are used, wherein the antibody is directed against the epitopes of the analyte to be detected. The now formed analyte-antibody-conjugate complex continues to flow through the NC membrane. The complexes formed are captured on the support membrane by immobilized antibodies at the signal line. This produces a colored band on the NC membrane. The coloring is dependent on the concentration of the analyte. The excess fluid with the non-bound antibody conjugate continues to flow through the membrane system until it reaches a filter paper, the wicking pad, where it is being absorbed. Thus a back flow is prevented. The structure of an antigen being captured by two antibodies is called a sandwich immunoassay

To display the correct performance of the assay, a strip test usually is equipped with an additional control line as an internal reference. Antibody conjugates which are not captured at the signal line flow thru to the control line where they are held. Depending on the quality of available recognition structures (antibodies) detection limits in the lower ng​​/ml range are achieved in whole blood or urine samples.
 
 

Application

Lateral flow tests provide a simple and cost-effective alternative to traditional laboratory methods. Their main advantage lies in the efficiency and speed of analysis. LFAs are used for screening of food pathogens, in medical practice and during home visits, in rehabilitation clinics and ambulances for the rapid detection of biomarkers in infarction diagnosis, assessment of inflammation values ​​and rapid detection of pathogens. In the detection of drugs LFAs also play a crucial role. Because this system provides optimized media, requires no investment and minimal training of the responsable staff. Low labor and material costs can help to make this platform a cost-effective screening tool. Because of its very simple handling without additional equipment qualitative and semi-quantitative information can be given in a short time. In conjunction with appropriate measuring instruments quantitative analyzes can be created.
 
 

Your entry into the Lateral Flow Technology

The use of lateral flow test strips and cassettes offers an easy entry into the world of immunoassays. In our shop you can find more information and available biomarkers on this assay format.

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