Flow-through cassette.Development of flow-through tests date from the early 1980s. These represent one of the earlier rapid test formats and are usually supplied in kits as individual cassettes with extraction and wash buffers included.

The test principle involves a flow of fluid containing the analyte through a porous membrane and into an absorbent pad. A second layer, or submembrane, inhibits the immediate backflow of fluids, which can obscure results. The analyte is captured on the surface of the membrane by analyte capture molecules and then visualized by the addition of analyte detection molecules. These tests can be used to detect both antibodies and antigens. To perform the test, a sample is applied to the membrane and allowed to wick through by capillary action.

Basic components
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Thereafter, sequentially, there is a wash step, addition of the signal reagent, and a second wash to clear the membrane. (See "How it works" for more information on how flow-through tests are performed and how they work.)

An advantage of this test type is that it is a very rapid test procedure, with results available in as few as 3 to 5 minutes. However, the tests need to be performed individually or in small batches and require constant attention. These are not “walk-away” tests like the lateral-flow test.

Test sensitivity is good for antibody detection (serology) assays, but detection of antigen is often less sensitive than lateral-flow or traditional enzyme immuno assay (EIA) methods.