Lateral flow test

Dyed particles also are commonly used in diagnostic lateral flow tests (like the common home pregnancy test), as the colors can be seen with the eye without the need for special detectors. In this type of assay, antibodies or antigens are coupled to the dyed particles and a sample solution applied to the test strip carries them along within a membrane. The particles then are captured at points in the membrane that represent either a control or a positive sample result. Large numbers of color particles docking at these points within the membrane create the visual lines associated with these disposable tests. 

Wang, X., K. Li, D. Shi, et al. 2007. Development of an Immunochromatographic lateral-flow test strip for rapid detection of sulfonamides in eggs and chicken muscles. J. Agric. Food Chem. 55 2072-2078. 

The technology of the immunochromatographic test, also called lateral flow test or strip test, has been used for many years. However, its application to food safety, especially in mycotoxin testing is quite recent. The typical immunochromatographic test strip consists of a sample pad conjugate pad, membrane absorbent pad and adhesive backing. 

S. J., and Guo, J. Q. (2006) Development of an immunochromatographic lateral flow test strip for detection of -adrenergic agonist... 

Rapid lateral flow test strips considerations for product development. (Bedford, MA Millipore Corp., 2002)... 

Seal, J., Braven, H., Wallace, P. (2006) Point-of-care nucleic acid lateral-flow tests. EVD Technol. 

Bangs Laboratories, I. (1999) Lateral Flow Tests, http //www.bangslabs.com/ technotes/303.pdf Accessed... 

Fig. 1. Lateral flow tests are often constructed from a series of materials that sequentially overlap. The goal is to imbed all reagents so that a flowing sample rehydrates and moves all materials up a test strip. Analytes and reagents then interact in zones placed on the strip. The result is a rapid test that provides information easily visible to the eye.

Two specific examples are described to illustrate the construction of a typical lateral flow test. The first is for detecting a small molecular weight analyte, cotinine, which is a metabolite of nicotine. The second example is for detecting HIV antibodies using a test capture line formed by immobilizing recombinant antigens (from a commercial source) on the nitrocellulose membrane. 

Cotinine is the major metabolite of nicotine and is useful for the determination of tobacco smoke exposure. In this case, the test is designed to detect exposure to second hand smoke. Active smokers generally have very high levels of cotinine in their body fluids, and subjects exposed to second-hand smoke are expected to show considerably reduced levels of cotinine. To further define the test, saliva is used as the sample. This further complicates testing, as saliva is a complex mixture of mucous-submandibular gland fluids (-75%) and low viscosity-parotid gland fluids (-25%). The test kit will thus need a collection device that reliably delivers the oral fluid sample to the lateral flow test sample loading pad. 

There are numerous companies that supply materials and components necessary to construct lateral flow tests, and unfortunately, there may be considerable variability in the quality and performance of these materials and components. The most crucial material is the nitrocellulose membrane, which supports the capture zones that are interrogated when the test is read. 

Finally, before beginning work on a lateral flow test, some specialized equipment is needed to ensure precise delivery of reagents onto the lateral flow test materials. Specific procedures for using some of this equipment are described below in the examples. Note that the goal here is to describe equipment appropriate for the research bench. There are also companies that offer equipment capable of manufacturing large numbers of test strips in highly automated fashion. 

As with all assays, each reagent must be adjusted to optimize test performance. A lateral flow test that is visually read is complicated by the fact that the interpretation of test results is subjective, dependent on ambient conditions and the experience of the tester. For this reason, it is preferable to use an instrumented reader to interrogate lateral flow test results. Recently, new inexpensive readers have become available. One such device (Avagotech, Menlo Park, CA) utilizes an inexpensive CCD camera, such as those found in mobile phones along with simple electronics to capture images of the lateral flow strip and its capture lines. Other more sophisticated reader devices are also available. The costs for any reader are dependant upon the capabilities of the reader hardware and software. Newer systems are now under development for fluorescence and chemilumenescent analysis. This new generation of readers is expected to increase lateral flow test analytical capabilities considerably. 

Our laboratory has been using an Avagotech reader for evaluation of reagents for HIV and cotinine lateral flow assays. Figure 2. shows an example of results for a sample diluted in our prototype HIV antibody detection lateral flow test strips. The Avagotech reader allows the data to be plotted and coefficients of variation to be determined, making the screening more quantitative and less subjective. 

Fig. 3. This is a typical image of a lateral flow test as seen using the Avagotech RDx reader. The assay may be measured as a fixed end point as shown or kinetically monitored during test development...

This contains a list of physical components for lateral flow tests and their function... 

Dessicants Maintains low humidity while storing lateral flow tests... 

At the same time, much simpler yet very successfiil microfluidic analysis systems based on wettable fleeces emerged First very simple dipsticks for e.g. pH measurement based on a single fleece paved the way for more complex test strips that have been sold as lateral-flow tests in the late 1980s [14]. Examples that are still on the market today are test strips for pregnancy [15], drug abuse [16-18], cardiac markers [19] and also upcoming bio-warfare protection [20]. 

In lateral flow tests, also known as test strips (e.g. pregnancy test strip), the liquids are driven by capillary forces. Liquid movement is controlled by the wettability and feature size of the porous or microstructured substrate. All required chemicals are pre-stored within the strip. The presence of an analyte is typically visualized by a colored line. 

The first immunoassay performed in a capillary driven system was reported in 1978 [67]. Based on this technique, the commonly known over-the-counter pregnancy test was introduced into the market in the middle of the 80 s. Today, this microfluidic platform is commonly designated as a lateral flow test (LAT) [14]. Other terms are test strip , immunochromatographic strip , immunocapillary tests or sol particle immunoassay (SPIA) [68]. Astonishingly, hardly any publications from a microfluidic point of view or in terns of material classification exist, and apparently many company secrets are kept unpublished [69]. 

Figure 3. Schematic design of a lateral flow test (According to [69]) (a) sample pad (sample inlet and filtering), conjugate pad (reactive agents and detection molecules), incubation and detection zone with test and control lines (analyte detection and functionality test) and final absorbent pad (liquid actuation), (b) Start of assay by adding liquid sample, (c) Antibodies conjugated to colored nanoparticles bind the antigen, (d) Particles with antigens bind to test line (positive result), particles w/o antigens bind to the control line (proof of vahdity).

Lateral flow tests were among the first successfully commercialized micro-fluidic products. A huge amount of assays has been developed on the capillary test strip platform during the past 30 years [76]. Today, they serve a wide... 

In total, the lateral flow test is a well established platform with a large but limited field of applications and consequently a benchmark for the home-care and IVD sector in terms of cost per assay and simplicity. 

Compared to lateral flow tests, this principle was one step ahead in result quantification and possible applications, but also in complexity of the processing device and disposable. 

R-Biopharm RIDA Lateral flow test, QUICK Gliadin qualitative immuno-chromatographic test... 

All the tests are designed for screening, and as such should be used only for preliminary screening for the presence of the nut residue. The validity of results obtained with the tests should preferably be viewed in conjunction with data from a validated laboratory assay (Table 20.8). Irrespective of the shade of a line observed in any of the lateral flow tests, a response is recorded (i.e., a positive for a test line appearing in two-line tests to indicate the presence of nut proteins). Additionally, a negative test result cannot exclude the possibility that the food contains the nut proteins because they are either distributed unevenly or may be below the detection limit of the test. 

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