A lateral flow test (LFT) works by using capillary action to draw a sample across a test strip, where it interacts with antibodies to indicate the presence of a specific substance.
Here's a breakdown of the process:
-
Sample Application: A sample, such as a nasal swab for a COVID-19 test or urine for a pregnancy test, is applied to a designated area on the test strip.
-
Sample Pad: The sample flows into the sample pad, where it mixes with specific antibodies that are tagged with colored particles (usually gold). If the target substance (antigen or hormone) is present, it binds to these tagged antibodies.
-
Conjugate Pad: The mixture then migrates to the conjugate pad. Here, more antibodies are present. The target substance, now bound to the tagged antibodies, binds to these antibodies.
-
Test Line: The mixture continues to flow to the test line. This line contains further antibodies that are specific to the target substance. If the target substance is present, the antibody-antigen-antibody complex will bind to the antibodies on the test line, causing a visible colored line to appear. The color comes from the accumulation of the tagged antibodies.
-
Control Line: Finally, the mixture flows to the control line. This line contains antibodies that bind to the tagged antibodies, regardless of whether the target substance is present. This ensures the test has worked correctly. A colored line must appear in the control line for the test to be valid. If no line appears in the control line, the test is invalid, and should be repeated.
-
Capillary Action: The entire flow of the liquid sample across the test strip is driven by capillary action, where the liquid is drawn along the porous membrane without needing any external force.
In essence, the lateral flow test relies on the specific binding of antibodies to the target substance. The appearance of colored lines indicates the presence (test line) and proper function (control line) of the test.
