The latex method, also known as the colloidal gold method, refers to a detection method using colloidal gold as a tracer. It is one of the approved new coronavirus antibody detection methods by the National Medical Products Administration in China.
Colloidal gold is formed by the polymerization of chloroauric acid under the action of reducing agents such as white phosphorus, ascorbic acid, sodium citrate, and tannic acid, to form gold particles of a certain size. Due to electrostatic effects, it becomes a stable colloid state and forms a negatively charged hydrophobic colloid solution. Colloidal gold is also an ideal immune marker in immunoelectron microscopy.
The test results are only for clinical reference and should not be the sole basis for diagnosing new coronavirus infections. If the test result is positive, further nucleic acid testing is recommended for confirmation.
The colloidal gold method determines whether the body has produced antibodies against the new coronavirus to judge whether the body is infected with the virus. A positive result means that there are corresponding antibodies in the body, while a negative result means that there are no corresponding antibodies. Since the human body's immune response takes time, for people who have just been infected with the new coronavirus, there may be a possibility of false negatives (i.e., misdiagnosis) because there are no corresponding antibodies produced in their bodies yet.
Colloidal gold technique is a technology that combines antigen-antibody immune reaction with colloidal gold labeling and tracking technology for qualitative and quantitative detection of antigen and antibody content. Due to its advantages of rapidity, simplicity, low cost, and good stability, colloidal gold detection has been widely used in the field of clinical testing.
At present, the commonly used detection methods for colloidal gold detection platforms include sandwich ELISA, competitive assay, indirect assay, etc. Different detection methods have different detection principles and substances suitable for detection. The following will introduce these methods separately.
Sandwich ELISA is the most commonly used detection method in the colloidal gold method testing platform, mainly used to detect larger biomolecules and particulate antigens. It requires preparing paired antibodies for the antigen to be tested, with one antibody labeled with colloidal gold and fixed on the binding pad, and the other antibody fixed on the test line (T line) of the nitrocellulose (NC) membrane. In addition, a secondary antibody that specifically binds to the gold-labeled antibody is prepared and fixed on the control line (C line) of the NC membrane.
Therefore, when two red lines are shown on the kit, it indicates that the tested substance is positive in the sample, while when only one red line is shown, it indicates that the tested substance is negative. The higher the concentration of the substance, the stronger the color intensity of the T line.
It is difficult to prepare paired antibodies for small molecule antigens (due to their small molecular size, it is difficult to find two binding sites for two antibodies to bind simultaneously), so sandwich ELISA cannot be used to detect small molecule antigens. For small molecule antigens, competition assay is commonly used. In the colloidal gold method testing platform using competition assay, the gold-labeled antibody is fixed on the binding pad of the kit, the T line on the NC membrane is fixed with a large molecule coupled to the antigen to be tested (since small molecule antigens cannot be directly fixed on the NC membrane, they need to be coupled to large molecule substances such as BSA for fixation on the NC membrane), and the C line is fixed with a secondary antibody that specifically binds to the gold-labeled antibody.