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Colloidal gold technology is a technology that combines antigen-antibody immune response with colloidal gold labelling technology for the qualitative and quantitative test of antigen and antibody content, due to its advantages such as rapidness, simplicity, low cost, and good stability. The commonly used test methods of colloidal gold antibody test platform include double antibody sandwich method, competition method, indirect method, etc. Different test methods have different test principles and different substances suitable for the test. These methods are respectively introduced below.

Ⅰ. Colloidal gold method: double anti-sandwich method

The double-antibody sandwich method is the most commonly used in vitro diagnostic solutions for colloidal gold test platforms, mainly used to test relatively large biomolecules and particulate antigens. The double-antibody sandwich method requires the preparation of the paired antibody of the antigen to be tested. One antibody is labelled with colloidal gold and fixed on the bonding pad, and the other antibody is fixed on the test line (T line) of the NC membrane. In addition, it is necessary to prepare a secondary antibody that specifically binds to the gold-labelled antibody (i.e., colloidal gold-labelled antibody) and fix it on the control line (C line) of the NC membrane. Therefore, when two red lines are displayed on the colloidal gold test strip, the substance to be tested in the sample is positive; when there is only one red line on the reagent strip, it means that the significance to be tested in the model is negative. The higher the concentration of the substance to be tested, the stronger the T-line's colour intensity.

Ⅱ. Colloidal gold method: competition method

Small molecule antigens are challenging to prepare paired antibodies (the molecular weight is small, and it is difficult to find two binding sites for two antibodies to bind simultaneously), so small molecule antigens cannot be tested by the double antibody sandwich method. For small-molecule antigens, the competition method is usually used for testing.
Using the colloidal gold test platform of the competition method, the gold-labeled antibody is fixed on the binding pad of the reagent strip, and the T line on the NC membrane is set with the macromolecule-conjugated antigen to be tested. Selected on line C is a secondary antibody that specifically binds to the gold-labelled antibody. Therefore, if there are two lines on the reagent strip, the substance to be tested in the sample is negative; if there is a line on the colloidal gold test strip, it means that the substance will be tested in the model is positive. The higher the concentration of the test substance in the sample, the weaker the T-line color intensity.

Ⅲ. Colloidal gold method: the indirect method

The indirect method in the colloidal gold method is mainly used to test antibodies. Suppose the colloidal gold test platform uses the indirect method. In that case, the binding pad of the reagent strip is fixed with protein A labelled with colloidal gold (Protein A can bind to the antibody non-specifically). The T line on the NC membrane can be fixed with an anti-protein A antibody that is immobilized on the C line on the NC membrane to the antigen bound explicitly by the antibody to be tested.

Therefore, if there are two lines on the reagent strip, the substance to be tested in the sample is positive; if there is a line on the colloidal gold test strip, it means that the substance will be tested in the sample is negative. The higher the concentration of the analyte in the sample, the stronger the T-line color intensity.

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