Enzyme-linked assay is an important method of immunoassay using antibodies and antigens that is common in clinical practice. Enzyme-linked assay has the advantages of high sensitivity, high specificity, easy to record and save the results, etc. It can be applied to a large number of tests, and has high requirements for the testing personnel. The principle is to let the antibody and enzyme complexes in the detected material bind, at which time the enzyme and the substrate produce a color reaction, and then detected by the color development reaction. Currently, enzyme-linked assays are widely used in clinical settings, such as monitoring HIV (Human Immunodeficiency Virus) antibodies, Hepatitis B Virus Antigen, Tetanus Antitoxin, and so on. Enzymes are widely used in clinical testing by designing important assays suitable for different types of conditions through different substrates and enzyme conjugates.