The composition principles of HCG pregnancy test products are mainly based on antigen-antibody reactions and some specific signal display mechanisms. The following is a detailed introduction:

Pregnancy Test Kits and Pregnancy Test Strips

• Main Composition: They usually consist of a water absorption layer, a reaction layer, and a display layer, etc. The reaction layer contains antibodies that can specifically bind to hCG, and there is generally an area with two lines, namely the test line (T line) and the control line (C line).
• Principle
  • Specific Binding of Antigen and Antibody: After a woman becomes pregnant, the hCG in her urine will be excreted with the urine. When the urine comes into contact with the water absorption layer of the pregnancy test strip or pregnancy test kit, it will diffuse upward along the strip to the reaction layer. The antibodies in the reaction layer can specifically recognize and bind to hCG to form an antigen-antibody complex.
  • Color Development Principle: Different substances are fixed at the test line and the control line. Another antibody against hCG is fixed at the test line. When the urine containing hCG passes through, hCG will bind to the antibody at the test line, forming a “antibody-hCG-antibody” sandwich structure. If hCG is present in the urine, the test line will develop color due to the formation of this sandwich structure. At the control line, a substance that can bind to an irrelevant antibody is fixed. Whether there is hCG in the urine or not, a certain amount of antibody will bind to it and develop color, which is used to determine whether the test is valid.

Blood hCG Test Reagents

• Main Composition: Generally, it contains components such as a reagent-coated plate, an enzyme-labeled substance, and a substrate. Antibodies that can bind to hCG are fixed on the reagent-coated plate.
• Principle
  • Double Antibody Sandwich Enzyme-linked Immunosorbent Assay (ELISA): During the test, the blood sample is added to the reagent-coated plate, and the hCG in the sample will bind to the antibody on the coated plate. Then, another enzyme-labeled hCG antibody is added, which will bind to the hCG that has already bound to the coated plate, forming a “coated antibody-hCG-enzyme-labeled antibody” sandwich complex. After that, the substrate is added, and the enzyme will catalyze the substrate to produce a color reaction. The content of hCG in the blood is determined by detecting the intensity of the color. The darker the color, the higher the concentration of hCG in the blood.
  • Chemiluminescence Method: Similar to the ELISA method, it also uses the double antibody sandwich principle. The difference is that instead of an enzyme, a substance that can generate chemiluminescence is labeled. After the sandwich complex is formed, the content of hCG is quantitatively analyzed by detecting the intensity of chemiluminescence. This method has higher sensitivity and can detect lower concentrations of hCG.