Written and maintained by CASRAI Editorial Board
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This page describes products and general use; it is not medical advice.
A lateral flow test is a device in which a liquid sample moves along a porous strip by capillary force and meets antibodies that signal whether a target substance is present. This page is for clinic staff, buyers and students who need to know what the strip is made of, what each part does, and which assay format a given test uses.
What a lateral flow test is
A lateral flow test (also called a lateral flow assay, LFA, or lateral flow immunoassay, LFIA) is an antibody-based test strip. A review in Essays in Biochemistry notes that the liquid flows across the device because of the capillary force of the strip material, so no pump or instrument is needed to move the sample. The same review describes these tests as widely used in hospitals, physician’s offices and clinical laboratories for detecting specific antigens and antibodies, and says the format suits primary screening at the point of care.
Who uses it: clinics, physician’s offices and clinical laboratories running point-of-care screening, as well as food-safety and environmental testing. Specific commercial kits, such as the rapid strep test, build on this format; a test sold for use in a clinic is also an in vitro diagnostic, and some are categorised as CLIA-waived tests.
How the strip is built
Both reviews cited below describe the same layered design: several overlapping pads and a membrane mounted on a backing card (a plastic backing) for stability and handling. The sample moves in one direction, from the sample pad at one end to the absorbent pad at the other.
| Part | What it does | Source |
|---|---|---|
| Sample pad | Receives the sample. It is usually impregnated with buffer salts, proteins and surfactants that control flow and make the sample suitable for the detection system; its most important role is to distribute the sample evenly and direct it to the conjugate pad. | Koczula and Gallotta 2016 |
| Conjugate pad | Holds the detector particles (the labelled antibody or antigen) and keeps them functionally stable until the test is run; the sample mixes with the conjugate here. | Koczula and Gallotta 2016; Law et al. 2015 |
| Nitrocellulose membrane | The porous membrane carrying the test and control lines. The review calls it the most critical element of the strip and nitrocellulose by far the most commonly used material. | Koczula and Gallotta 2016 |
| Absorbent pad | Wicks the fluid through the membrane and collects the processed liquid; it also allows larger sample volumes. | Koczula and Gallotta 2016 |
| Backing card | Holds the overlapping membranes together for stability and handling. | Koczula and Gallotta 2016 |
The detector label
The coloured particle on the conjugate is the label. Koczula and Gallotta state that colloidal gold is the most widely used label in commercial lateral flow immunoassays. Law and colleagues list monodisperse latex, colloidal gold, carbon and fluorescent tags as labels used in these devices.
Sandwich and competitive formats
Two basic formats exist, and the analyte decides which one a test uses. Direct (sandwich) tests suit larger analytes or analytes with several antigenic sites; competitive tests suit small molecules with a single antigenic site that cannot bind two antibodies at once.
| Format | Analyte it suits | How the test line works |
|---|---|---|
| Sandwich (direct) | Larger analytes and analytes with multiple antigenic sites or epitopes | The target is held between two complementary antibodies, so the labelled conjugate builds up on the test line when the analyte is present. |
| Competitive | Small molecules with a single antigenic determinant (single epitope) | The analyte blocks binding sites on the test-line antibodies and prevents the coloured conjugate from attaching there, so the line behaves in the opposite direction to a sandwich test. |
Sources: Koczula and Gallotta 2016; Law et al. 2015. In both formats, a response on the control line shows that the liquid flowed properly through the strip; it does not by itself describe the sample.
What the format can and cannot do
- Output is on/off or semi-quantitative. Koczula and Gallotta say results are mostly visual, qualitative (on/off) or semi-quantitative, and can be read by eye or with a dedicated reader.
- Screening, not confirmation. The same review says regulatory bodies often require confirmation with an independent method, so the format is suited to primary screening at the point of care or need.
- One test at a time. Law and colleagues describe lateral flow immunoassays as simple and fast but designed for individual tests rather than high-throughput screening.
- Cross-reactivity is possible. Because the test relies on antibodies, other chemicals with a similar structure can affect specificity and sensitivity, leading to false positives.
- Storage. The review notes a long shelf life and that refrigeration is not required for storage of these tests.
Safe use and handling basics
This page does not replace the instructions for use (IFU) supplied with a specific test. The strip design above is generic; sample type, volumes, buffers, timing and how to read the lines differ between products, so follow the manufacturer’s IFU for the exact test. Interpreting a particular result is outside the scope of this page.
Common mistakes
- Treating the control line as a verdict on the sample. It indicates proper liquid flow only.
- Assuming every lateral flow test works the same way. Sandwich and competitive formats respond differently on the test line, so read the IFU for the product’s format.
- Expecting precise measured values from a format that is mostly on/off or semi-quantitative.
FAQ
How do lateral flow tests work?
The sample is applied to the sample pad and flows by capillary force through the conjugate pad, where it meets the labelled antibodies, then across the nitrocellulose membrane past the test and control lines, and finally into the absorbent pad.
What is a lateral flow test strip made of?
A sample pad, a conjugate pad, a nitrocellulose membrane and an absorbent pad, mounted on a backing card. See the parts table above.
What are the lateral flow assay components that detect the target?
Antibodies (or antigens) carried by the label on the conjugate pad and immobilised on the test line of the membrane. Colloidal gold is the most widely used label in commercial tests.
What is the difference between sandwich and competitive formats?
Sandwich tests fit analytes with several binding sites; competitive tests fit small analytes with a single binding site. See the format table above.
Sources
- Koczula KM, Gallotta A. Lateral flow assays. Essays in Biochemistry 2016;60(1):111-120. europepmc.org/article/PMC/PMC4986465 (checked 2026-10-30).
- Law JW-F, Ab Mutalib N-S, Chan K-G, Lee L-H. Rapid methods for the detection of foodborne bacterial pathogens: principles, applications, advantages and limitations. Frontiers in Microbiology 2015;5:770. europepmc.org/article/PMC/PMC4290631 (checked 2026-10-30).
Related reading: direct ELISA assay, another antibody-based assay format, and throat culture swabs compared with rapid test swabs.
Related supplies: diagnostic, monitoring and imaging supplies.
