An immunochromatographic assay, commonly known as a lateral flow test, is a device that detects a specific substance in a liquid sample. This technology enables rapid testing without specialized equipment, providing results in about five to thirty minutes. These tests are widely used for medical diagnostics at home and in clinical settings. The home pregnancy test, for instance, is a lateral flow test that detects a specific hormone in urine.
The Mechanism of a Lateral Flow Test
A lateral flow test uses a strip of overlapping materials. The process begins when a liquid sample is applied to the sample pad, a porous section that absorbs the fluid. From here, the liquid moves via capillary action, being drawn along the material without external force.
The fluid flows to the conjugate pad, which contains millions of microscopic particles, often red colloidal gold, coated with antibodies specific to the target antigen. These freeze-dried particles mobilize when the liquid sample reaches them. If the sample contains the target antigen, it binds to these colored antibodies, forming a complex.
This mixture migrates onto a nitrocellulose membrane containing two lines. The first is the test line, with immobilized antibodies also specific to the target antigen. As the complex flows past, these capture antibodies bind to it, causing the colored particles to accumulate and form a visible line for a positive result.
Further down is the control line, which contains antibodies that bind directly to the colored particles from the conjugate pad, regardless of antigen presence. A line must appear here to confirm the test is working correctly. The fluid collects in an absorbent wicking pad at the end, which prevents backflow.
Most rapid tests use a sandwich assay format, ideal for larger molecules like the hCG hormone. In this design, the target antigen is “sandwiched” between the colored antibody and the capture antibody on the test line. A less common format is the competitive assay, used for smaller molecules. Here, a visible line appears when the target is absent from the sample.
Everyday and Clinical Applications
The simplicity and speed of lateral flow tests have led to their widespread adoption in numerous settings far beyond the familiar pregnancy or COVID-19 tests. Applications include:
- Medical diagnostics: To rapidly identify infectious diseases like influenza, strep throat, and malaria, or to detect cardiac markers in patients with suspected heart attacks.
- Veterinary medicine: To screen for common animal diseases, such as feline leukemia virus in cats or canine parvovirus in dogs, allowing for quick, in-clinic answers.
- Food safety: To detect contaminants like bacterial toxins or allergens such as peanuts and gluten in food products.
- Environmental monitoring: To screen water sources for pollutants or harmful algal blooms.
- Substance abuse screening: To test urine samples for the presence of various drugs.
Understanding Test Reliability
The reliability of a lateral flow test depends on two main performance characteristics: sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify individuals who have the target substance. A test with high sensitivity will have very few false negatives, meaning it is unlikely to miss a positive case.
Specificity, on the other hand, is the test’s ability to correctly identify individuals who do not have the target substance. A highly specific test will have very few false positives, meaning it is unlikely to indicate the presence of a substance that is not actually there. Both metrics are determined by comparing the lateral flow test’s performance against a more established laboratory method.
Despite being powerful tools, test results can sometimes be inaccurate. User error is a common cause, such as collecting a sample improperly or misinterpreting the results. The timing of the test is also a factor; testing too early in an infection may result in a false negative because the concentration of the antigen is still too low to be detected.
In some instances, the chemistry of the test itself can lead to incorrect results. A false positive can occur due to cross-reactivity, where the test’s antibodies mistakenly bind to a molecule that is structurally similar to the target antigen. A false negative could happen if other substances in the sample interfere with the binding mechanism.