A lateral flow assay (LFA) is a diagnostic device designed to detect specific substances in a liquid sample. These devices are lightweight, portable, and provide results quickly, often within 5 to 30 minutes. Their design makes them suitable for both clinical and at-home use, offering a convenient way to test for various conditions using samples like saliva, urine, or blood.
How a Lateral Flow Test Works
The operation of a lateral flow test relies on the movement of liquid across a series of absorbent pads, driven by capillary action. The test is built on a plastic backing card holding four main components that overlap to ensure the sample flows continuously. The liquid sample is applied to the first component, the sample pad, which is often treated to filter particulates and adjust the sample’s pH.
From the sample pad, the liquid flows into the conjugate pad. This pad holds millions of microscopic colored particles, typically gold, bonded to antibodies designed to bind to a specific target substance. The liquid sample rehydrates these dried antibody-particle conjugates. If the target substance is present, it binds to these antibodies, creating a mobile complex.
This mixture then travels onto a nitrocellulose membrane, which contains two distinct lines: the test line (T) and the control line (C). The test line contains fixed antibodies that also target the substance being tested for. As the liquid flows past, these immobilized antibodies capture the complexes, causing the colored particles to accumulate and form a visible line, indicating a positive result.
Further down the strip is the control line, which contains antibodies that bind directly to the colored nanoparticle conjugates, regardless of whether they have captured the target. This line must appear for the test to be valid, confirming the liquid flowed correctly and the reagents are active. Any remaining liquid is drawn into the absorbent pad at the end of the strip, which maintains the flow and prevents backflow.
Common Uses of LFA Tests
The applications for lateral flow assays are extensive. In clinical settings, they are used for the rapid detection of infectious diseases and in reproductive health. Common uses include testing for:
- Viruses like influenza and SARS-CoV-2
- Bacteria such as Streptococcus (strep throat)
- Mosquito-borne diseases like malaria
- Reproductive hormones like human chorionic gonadotropin (hCG) for pregnancy or luteinizing hormone (LH) for ovulation
Beyond human health, LFA technology helps ensure food safety. These tests can quickly screen for allergens, such as peanuts or eggs, in food products to prevent allergic reactions. They are also employed to detect harmful pathogens like Salmonella or toxins in food, providing a rapid method for quality control.
The versatility of LFA tests also extends to environmental monitoring. Government agencies and environmental organizations use these devices to test water supplies for various contaminants and pollutants. This application provides a fast, on-site method for assessing water quality in rivers, lakes, and drinking water systems.
Understanding Test Results and Reliability
Interpreting lateral flow test results is straightforward. A positive result is indicated by the appearance of two lines at the control (C) and test (T) positions. A negative result shows only the control line, confirming the test worked but did not detect the target. An invalid result occurs if the control line does not appear, meaning the test failed and its result cannot be trusted.
The reliability of a diagnostic test is measured by its sensitivity and specificity. Sensitivity is the test’s ability to correctly identify individuals who have the target substance, minimizing false negatives. Specificity is the ability to correctly identify those who do not have the substance, minimizing false positives. LFAs offer speed and convenience in exchange for slightly lower sensitivity than lab-based tests like PCR.
This difference in sensitivity is important. A false negative can occur with an LFA, particularly in the early stages of an infection when the concentration of the target substance is low. Because of this, the timing of the test can influence its accuracy. While LFA tests are highly specific and false positives are less common, a confirmatory lab test may be recommended depending on the context.