Sample Pad and Conjugate Pad For Lateral Flow Assays

Lateral immunoassay (lateral flow immunoassays) is a main form of point of care test (POCT), its essence is still the immune response of antibody antigen. However, the traditional method is to set the antibody antigen reaction on a 96-well plate, or on the surface of magnetic microspheres, while lateral immunochromatography detection places the immune reaction in a porous nitrocellulose membrane (NC membrane) Therefore, it is also called test strip detection.

The main components of lateral flow immunoassay detection include absorbent paper, chromatographic membrane, conjugate pad and sample pad, and a base plastic plate supporting all materials. The liquid sample is added from the sample pad and flows toward the side of the absorbent paper. When passing through the conjugate pad, the immunoprobes (colloidal gold, fluorescent microspheres, etc.) on the conjugate pad are released, and react with the analyte in the sample during the flow process, and then fixed to the detection line on the NC membrane (Test Line) The antibody captures and aggregates to form an optical signal.

Lateral flow assays originated in the 1990s. With the stability of colloidal gold preparation technology, a rapid diagnostic method developed in conjunction with immunoassay has developed. According to the development of probes, the lateral flow test technology has gone through three main stages: colloidal gold, dyed latex microspheres, and fluorescent microspheres. Correspondingly, the detection results have also gone through three stages: qualitative, semi-quantitative, and quantitative.

The sample pad can be used to perform multiple tasks, the most important of which is to promote a uniform and controlled distribution of the sample on the conjugate pad. It also controls the rate at which liquid enters the conjugate pad, thereby preventing the device from overflowing. When impregnated with proteins, detergents, tackifiers and buffer salts.
There are generally two types of materials used as sample pads: cellulose fiber filters and woven mesh.

By adding a retarder to the sample pad, the retardation of the membrane can be eliminated. This method may be easier and more cost-effective than trying to directly block the membrane. Unless the antibody (or antigen) is covalently attached to the detector particles, in the presence of blocking proteins or detergents, it is recommended not to dry the detector reagents into the binding pad.

Increase sample viscosity (adjust flow characteristics).
Enhance the ability of samples to dissolve detection reagents.
Prevent conjugates and analytes from non-specifically binding to any downstream materials.
Modify the chemical properties of the sample to make it compatible with the immune complex on the test line.
Promote uniform flow of sample along the membrane.
Supplementation of protein and detergents and surfactants can promote the re-conjugation of conjugates, reduce the binding of non-specific binders, and may Minimize the analyte adsorption membrane.

Conjugate pads can perform multiple tasks, the most important of which is the uniform transfer of detection reagents and test samples to the membrane. When the sample flows into the conjugate pad, the detection reagent dissolves, lifts the pad material, and moves into the membrane together with the sample.
The important function of the conjugate pad is to deliver the test agent particles as a constant volume of sample on each test strip to the membrane.

Low non-specific binding. If the detection reagent or analyte binds to the binding pad, it will not be able to form immune complexes on the test line, thereby reducing signal strength and sensitivity.

Consistent flow characteristics. Inconsistent flow characteristics can cause serious performance problems. If the material fails to deposit the sample uniformly on the membrane, the detection reagent may be directed onto the membrane and streaks may appear as the sample migrates along the membrane. Therefore, uneven signals will appear on the test and control lines.

The bed volume is consistent. When loading it into a conjugate pad by immersion, the amount of detection reagent in each test strip depends on the bed volume of the material. If the volume of the bed varies greatly, a variable signal strength will be observed even if all other components of the strip remain unchanged.

Low extract. In addition to chemical extracts, the material should also be free of particles that can block the membrane at the conjugate pad / membrane interface.

Good web processing characteristics and consistent compressibility. These attributes are very important for incorporating test strip production and stably transferring liquid to the membrane.

Both are used to store different samples. The conjugate pad is a carrier for labeling proteins, while the sample pad is a carrier for pretreatment of biochemical solutes. Therefore, considering the long-term coexistence with the protein, the conjugate pad can only maintain a neutral pH, low salt ion concentration, etc., to create a good preservation environment for the protein. The sample pad is faced with a more complex sample solution, so it requires greater pretreatment and filtration capabilities.

From the perspective of labeling technology, lateral flow assay has gone through several stages such as colloidal gold, stained latex microspheres, and fluorescent microspheres. In addition, quantum dot fluorescence and magnetic particles have been developed in recent years. Colloidal gold and latex microspheres are mainly judged by the naked eye for color change, generally only for qualitative detection, and need to cooperate with the instrument to achieve semi-quantitative. With the help of fluorescence signal amplification and fluorescent analyzer to read the fluorescence intensity, fluorescent microspheres can already achieve quantitative detection, which is the mainstream of today's development.

The lateral flow immunochromatographic is a rapid immunoassay method that combines immunology and chromatography. It is mainly used for the detection of pathogens, hormones, drugs, and metabolites. The principle is to identify antigens or antibodies by labeling Inspection object.
BALLYA is committed to the research of biological sciences and has his own profound experience and experience in the use of lateral immunochromatography. Using lateral immunochromatography, Ballya has successfully developed a variety of rapid test kit for dairy products, honey, aquatic products, pesticides, adulteration, pet diseases and livestock diseases. In the future, Ballya will continue to conduct more in-depth research on more aspects of testing.