Lateral flow tests have different names in different countries and different sectors. Some of the most common names are:
- Lateral flow device (LFD)
- Lateral flow test (LFT)
- Lateral flow immunoassay (LFIA)
- Lateral flow assay (LFA)
- Lateral flow immunochromatographic assays
- Express test
- Quick test
- Rapid test
- Pen-side test
- Test strip
What Does Lateral Flow Immunoassay Mean?
Probably you have heard of lateral flow immunoassay and been wondering what could it be. This part covers all what it entails. A lateral flow immunoassay is a diagnostic device used in confirming the absence or presence of a target analyte. These analytes could be pathogens or biomarkers in animals or humans or contaminants present in animal feeds, water supplies or foodstuffs. Pregnancy test is the most popular lateral flow rapid test strip.
LFDs contain a control line used to determine if a test is working correctly together with one or more target lines. These LFDs are made to incorporate intuitive user protocols and need little to no training to operate. LFDs could be read visually and qualitative or give data when blended with fascinating reader technologies.
Lateral tests are employed in point-of-care testing for the human health. These tests can be done by healthcare practitioners or by patients in different settings such as the laboratory, home, or at the clinic. Medical diagnostic industry has some strict regulatory requirements that must be followed for all manufactured and developed products. North American Diagnostics provides a superb guideline on how to go about these products. Lateral flow rapid tests can be used in other industry sectors because of their flexible nature. These sectors may include plant and crop health, pharma, food and feed testing, environmental testing, and animal health.
LATERAL FLOW TEST TYPES
Lateral flow assays are created to be implemented in a housed cassette or a dipstick format. Both housed tests and dipstick tests work in almost the same manner. However, they are dependent on the sector they appear in, the market requirement, and the sample matrix to determine the most suitable format.
There are two assay types; competitive assay and sandwich assay.
Competitive assay (inhibition immunoassay format)-this assay represents a positive test but there is no colored line at the position of the test line. A line in the test region represents a negative test. Examples of this test format are tests for toxins and drugs. This test format is employed for small molecular weight analytes with single antigenic determinants.
Sandwich assay (non-competitive immunoassay format)-this assay represents a positive test in the form of a colored line at the position of the test line. A negative test is showed by the absence of a line in the test region. Examples of this test format are over-the-counter pregnancy tests. This format works for sizeable molecular weight analytes with multiple antigenic sites.
The non-competitive immunoassay has lower analytical sensitivity compared to the competitive format. Non-competitive formats suffer from a high-dose hook effect when subjected to a high analyte concentration, which leads to false results. Competitive format, on the other hand, does not experience the high-dose hook effect.
To determine the type of sample to use in an assay, the market requirements and target analyte must be considered. Some samples like the animal feed need running buffer to assist in sample delivery, while samples like serum, saliva, urine, or blood can be tested directly. In some occasions, a dilution buffer is more fruitful.
Some tests for the quantitative estimation of lambda immunoglobulin and kappa free light chains (FLCs), utilizes serum and still needs a buffer for the sample solution.
In other instances rapid antibody tests recognizes IgG antibodies that targets the stubborn SARS-CoV-2 spike protein, utilizing buffers.
TYPES OF LABELS
Lateral flow assays generally make use of conjugated carbon, gold, or colored latex nanoparticles in the combined pad for labeling. Other labels are colored polystyrene beads or magnetic beads.
Labels perform the same function regardless of what label types they are which is creating a three-way bond with targets and antibodies to make the test and control lines visible. Labels are selected depending on factors like sample matrix, antibody, and target. This label selection is done during lateral flow development to ensure assay optimization. The label interacts perfectly with the antigen and antibody to ensure accurate results and efficiency is obtained.
MULTIPLEXED LATERAL FLOW ASSAYS
The competitive and sandwich assays can both be customized to include one or several test lines. Our nucleic acid lateral flow immunoassay, PCRD, for example, is a unique assay with a single control line and two test lines. Competitive and sandwich multiplex assays can be implemented using complementary reader technology to produce quantitative results.
A multiplexed assay is used for the following purposes:
- Assisting diagnosis where several markers are required to be present.
- Confirming if multiple contaminants are present during high volume feed and food testing.
- Testing in remote areas with limited resources where multiplexed testing saves a lot of time.
- Offering cost-saving advantages to the final users in-the-field or in a laboratory by testing for different targets.
- Detecting multiple targets in one test instead of using several individual tests. In instances where a small sample volume is what is acquired, a multiplex assay will allow the users to maximize its use in testing.
QUANTITATIVE RAPID LATERAL FLOW DEVICES
Using titles like a quick test or rapid test can lead to misconceptions about lateral flow equipment where it may mean they will be limited in their capability.
Some myths about lateral flow devices are;
- They can only be qualitative.
- Their accuracy does not meet laboratory requirements.
- They are not flexible enough to meet evolving and growing diagnostic challenges.
However, lateral flow devices are very effective, easy to use, compact and they offer a considerable amount of flexibility. Earlier LFD types were mainly qualitative assays, but improvements in component materials, reagents, and reader technologies together with manufacturing processes imply quantitative results are attainable. Development in reader technology and raw materials like labels indicate that lateral flow rapid tests can counter-part the ELISA assay’s sensitivity.
HOW A LATERAL FLOW TEST WORKS
LFDs apply immunoassay technology by use of colored nanoparticles (or labels), nitrocellulose membrane and antibodies to get results. Upon the addition of a sample, the flow is expected along the lateral flow test device, via the conjugate pad and then directly into the nitrocellulose membrane. The flow ends on to the absorbent pad.
How a sandwich assay works:
- The sample pad is the first stage in the absorption process, which sometimes contains a filter to make sure a controlled and accurate sample flow happens.
- The conjugate pad containing antibodies and conjugated labels receives the sample. In case the required target is present, the conjugate labels and antibodies fuse with the target and continue migrating along the test.
- As the sample flows along the gadget, binding reagents found on the nitrocellulose membrane combine with the target where the test line appears. This forms a colored line whose density varies according to the amount of target present in the sample. Some targets require quantification to check the target concentration. To provide quantitative results, the rapid test is infused with a reader.
- After passing through the nitrocellulose membrane, the excess sample is absorbed into the absorbent pad. The choice of absorbent pad used will have some effect on the volume of the sample to be used.