Lateral Flow Rapid Diagnostic Tests Explained
Lateral flow immunoassays are a cornerstone of the rapid, point-of-care diagnostic test market. Commonly known from their early utility in home pregnancy test kits, lateral flow immunoassays are now used for a variety of devices designed for point-of-care settings without the need to send samples to a laboratory or clinic. Lateral flow tests can clearly detect the presence of a wide range of biomarkers, pathogens and environmental contaminants in a variety of industries.
Although the foundations of lateral flow technology are more than 30-years old, systems have evolved to meet the advanced diagnostic needs of today. Modern lateral flow tests enable high sensitivity and multiplexing, and they address the high error rate that often occurs at the point of sampling with simple user-centric designs. Lateral Flow devices can range from simple, low cost and lower complexity qualitative tests to complex designs involving the quantification of multiple biomarkers performed in the field without the need for capital equipment and infrastructure.
What Is a Lateral Flow Rapid Diagnostic Assay?
Lateral flow assays, often known as Rapid Test Devices or RDT’s are immunosensors that rely on the use of low cost materials to achieve the flow of reactants. At their simplest, they are dipsticks with a sample application pad at one end, a reading zone where the signal is generated in the middle and an absorbent material at the other end. Tests may be incorporated into housings (cassettes) that support higher functionality, branding and user-centric design goals.
Lateral flow assays may be qualitative (indicating the presence or absence of an analyte), semi-quantitative or fully quantitative. These quantitative assays are useful in situations where people need to know not only whether a substance is present, but also how much of it is present.
A lateral flow test contains several important components:
- sample pad
- conjugate pad
- nitrocellulose membrane
- absorbent pad
These materials, each serving one or more purposes, overlap onto one another and are mounted on a backing card using a pressure sensitive adhesive. Figure 1 shows a typical configuration for a lateral flow assay.
Figure 1: Typical Lateral Flow Test Strip Configuration
How Does a Lateral Flow Test Work?
When a test is run, a sample is added to a Sample Application Pad. Here, the sample is treated to make it compatible with the rest of the test. The treated sample migrates through this region to the Conjugate Pad. Here, a particulate conjugate has been immobilized, typically a colloidal gold or a colored, fluorescent, or paramagnetic monodisperse latex particle or cellulose nanobead. This particle has been conjugated to one of the specific biological components of the assay, either antigen or antibody depending on the assay format. The sample re-mobilizes the dried conjugate, and the analyte in the sample interacts with the conjugate as they both migrate into the next section of the strip. This zone, known as the Reaction Matrix is a porous membrane onto which the other specific biological components of the assay have been immobilized. These are typically proteins, either antibody or antigen that have been laid down in bands in specific areas of the membrane where they serve to capture the target and conjugate as they migrate up the strip. Excess reagents move past the capture lines and are entrapped in the Wick or absorbent pad. Results are interpreted on the Reaction Matrix as the presence or absence of lines and can be read either by eye or using a reader.
Assay formats are either sandwich (direct) or competitive (competitive inhibition) in nature, and the assay format can accommodate qualitative, semi-quantitative, or fully quantitative assays. Direct assays are typically used when testing for larger analytes with multiple antigenic sites, such as hCG, Dengue antibody or antigen or HIV. In this case, a positive result is indicated by the presence of a test line. Less than an excess of sample analyte is desired, so that some of the conjugated particles will not be captured at the capture line, and will continue to flow toward the second line of immobilized antibodies, the control line. This control line typically comprises a species-specific anti-immunoglobulin antibody specific for the conjugated antibody. Competitive formats are typically used when testing for small molecules with single antigenic determinants, which cannot bind to two antibodies simultaneously. In this format, a positive result is indicated by the absence of a test line on the reaction matrix. A control line should still form, irrespective of the result on the test line. The two formats are illustrated schematically in Figures 2a and 2b below.
Figure 2a: Direct Solid Phase Immunoassay
Figure 2b: Competitive Solid Phase Immunoassay
Why Consider Lateral Flow Immunoassays for Your Rapid Diagnostic Testing Needs?
Lateral flow immunoassays are easy to use. The user simply needs to dip the sample pad into the liquid they want to test and then lay the test flat or apply the sample directly to the cassette. If the test is positive, a colored or fluorescent line will show up on the test along with a control line. If the test is negative, only the control line will be present. For many years, women with no medical training have successfully used this kind of test to detect pregnancy or ovulation at home.
More advanced lateral flow tests provide additional benefits, including higher sensitivity, multiplexing and the ability to detect multiple biomarkers from a single finger stick. These advanced tests are useful for in-the-field testing.
Lateral flow rapid tests provide results quickly. In many cases, the test and control lines show up almost immediately with definitive results available within a few minutes. This rapidity is important in medical settings where doctors have limited time available to spend with each patient, as well as in manufacturing process quality control checks in pharmaceutical companies, highly sensitive human performance testing for athletes, and bioprocessing and GMO testing in agriculture and food safety.
DCN is the go-to company for the design and development of point-of-care diagnostic assays that have tough specifications, including:
- High Sensitivity
- Tough Sample Matrices
- Reader Integration
Our cross-functional team of assay development and engineering professionals allows us to assist you in the development of your entire rapid diagnostic test from concept to commercialization.
Our staff has unparalleled experience in developing the highest performing point-of-care assays in lateral flow markets, ranging from medical diagnostics to veterinary, food science, bio-defense, agricultural, and environmental health tests.
DCN operates under ISO 9001:2015 and ISO 13485:2016. All of our development programs are done under Design Controls, and all production activities for reagents, components, assays and electronics are performed under our EN 13485 system.