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Lateral flow methods

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p385-386

2026-11-05

193

Lateral flow methods

There is a growing trend in the immunoassay sector to develop point-of-care tests. The Syva Emit system  on small analysers such as the ETS, has allowed drug testing outside the laboratory for many years (Centofanti 1994). The use of inexpensive, easy-to-use single disposable cartridges or slides for a variety of drugs has accelerated this trend. Single-use cartridges are basically variants of a competitive ELISA assay run on a solid-phase strip rather than inside a well or tube. These tests use particles as the means of signal generation and are mostly used with antibodies that have been labelled with colloidal gold or coloured latex spheres. This technique was established for urine drug testing and can also be applied to serum, whole blood (using a suitable filter pad to separate red cells) and saliva (employing a suitable collection method).

Figure 1 illustrates the ‘lateral flow’ or ‘immuno-chromatographic assay’ process for a single analyte. As with ELISA, an antigen protein derivative is fixed to a solid-phase. In this case it is bound to a nitrocellulose membrane as a line or series of dots at a defined position (Fig. 1A). Labelled antibody (typic ally labelled with colloidal gold or coloured latex) is located in a pad that overlaps with the nitrocellulose membrane. When the pad is wetted with sample, the labelled antibody rehydrates and flows from the pad onto the membrane by capillary action. As the sample and rehydrated gold-labelled anti body flow along the membrane, any drug in the sample binds to the labelled antibody and inhibits it from binding to the immobilised drug on the membrane (Fig. 1C). If no drug is present in the sample, the rehydrated antibody binds to the immobilised drug and a visible line is formed (Fig. 1B). Separation of bound and free drug continues, as part of the assay process, with the movement of liquid across the membrane, leaving either bound material (a visible line) or free material (no visible line) to travel to an absorbent end-pad. As with ELISA, the amount of colour developed (or line formed at the site of the immobilised drug) is inversely proportional to the amount of drug present in the sample. The addition of a control line on the membrane provides an indication that sufficient fluid has passed through the length of the test strip and thus an indication that the test has been performed correctly. Control lines typic ally use anti-IgG antibodies to capture excess drug-specific labelled antibody. The addition of a number of drug-derivative lines in different positions on the nitrocellulose membrane together with suitable matching labelled antibodies allows the analysis of multiple drugs at the same time from a single sample. The majority of these tests are interpreted visually, and the reagents are titrated and optimised such that there is no colour or line present when the ‘cut off’ concentration of analyte is present. Recent developments have overcome the problems of visual interpretation associated with lateral flow tests. This has been achieved by developing a small portable reader based on digital imaging. The benefits are that the line can be determined more accurately, the result can be quantified if required and the subjectivity of visual interpretation is removed (Spiehler et al. 2000).

Figure 1 A competitive lateral flow immunoassay.