Pet rapid – test kits operate on various detection principles depending on the target of detection, such as viruses, bacteria, genes, or physiological indicators. The following are some common ones:

I. Immunochromatographic Assay

1. Principle Overview
   • This is one of the most widely used principles in pet rapid – testing. It is based on the specific binding reaction between antigens and antibodies. The test strip in the kit usually consists of components such as absorbent material, conjugate pad, test line (T – line), control line (C – line), and nitrocellulose membrane.
   • When the sample (such as blood, feces, oral swab, etc.) is added to the sample well of the test strip, the sample moves forward along the test strip under capillary action. If the sample contains the target antigen (for example, viral antigen), the antigen first binds to the specific antibody labeled with a marker (such as colloidal gold or fluorescein) on the conjugate pad.
2. Reaction Process and Result Judgment
   • This antigen – antibody complex continues to flow with the liquid. When it reaches the test line (T – line), if there is another specific antibody on the T – line that can bind to the target antigen, the complex will be captured and accumulate at the T – line. If the content of the target antigen in the sample is sufficient, a visible band will form at the T – line (for example, a red band appears when labeled with colloidal gold, and a fluorescent signal is detected at a specific wavelength when labeled with fluorescein).
   • At the same time, regardless of whether the target antigen is present in the sample, the labeled antibody will bind to the anti – marker antibody on the control line (C – line) during the flow process, forming another band. The presence of the C – line is used to verify whether the test strip is working properly. If the C – line does not show color, it indicates that the test strip may be invalid or there is an error in the testing process. In this case, the test result is unreliable, regardless of whether the T – line shows color.

II. Enzyme – Linked Immunosorbent Assay (ELISA) Principle

1. Principle Overview
   • ELISA is mainly used to detect antigens or antibodies in samples. The principle is to adsorb antigens or antibodies onto the surface of a solid – phase carrier (such as a microplate), and then add the sample to be tested. If the corresponding antibodies or antigens are present in the sample, they will bind to the antigens or antibodies on the solid – phase carrier.
   • Subsequently, an enzyme – labeled second antibody (for antigen detection) or an enzyme – labeled antigen (for antibody detection) is added to form a complex of “solid – phase antigen – antibody – enzyme – labeled second antibody” or “solid – phase antibody – antigen – enzyme – labeled antigen”.
2. Reaction Process and Result Judgment
   • After adding the enzyme substrate, the enzyme catalyzes the substrate to undergo a color – developing reaction. The absorbance value of the reaction wells in the microplate is detected using a colorimeter or other equipment. If the absorbance value is higher than the set critical value, it is judged as a positive result, indicating that the target antigen or antibody is present in the sample; if the absorbance value is lower than the critical value, it is a negative result.

III. Polymerase Chain Reaction (PCR) Principle

1. Principle Overview
   • PCR is mainly used to detect specific gene fragments in pet samples, such as viral genes or genes related to genetic diseases. It is a technique for in – vitro DNA amplification. First, the double – stranded DNA in the sample is unwound into single – stranded DNA by high temperature, a process called denaturation.
   • Then, at a lower temperature, primers (short DNA sequences complementary to the two ends of the target gene fragment) specifically bind to the single – stranded DNA template, and this step is called annealing. Subsequently, under the action of DNA polymerase, using the single – stranded DNA as a template, DNA synthesis starts from the 3′ end of the primer to form a new DNA strand, which is the extension process. After multiple cycles (usually 20 – 40 cycles), the number of target gene fragments increases exponentially.
2. Reaction Process and Result Judgment
   • The amplified products can be separated and detected by methods such as agarose gel electrophoresis. If a band corresponding to the size of the target gene fragment appears on the gel, it indicates that the target gene is present in the sample, which is a positive result; if no corresponding band appears, it is a negative result. Some PCR test kits also incorporate real – time fluorescence quantitative technology to quantitatively analyze the content of the target gene in real – time by detecting the intensity of the fluorescence signal.

IV. Biochemical Reaction Principle

1. Principle Overview
   • For test kits that detect pet physiological indicators (such as blood glucose, renal function indicators, etc.), the biochemical reaction principle is often used. Taking blood – glucose detection as an example, the glucose oxidase method is a common approach. Glucose oxidase can specifically catalyze the oxidation of glucose to produce gluconic acid and hydrogen peroxide.
   • Hydrogen peroxide reacts with a specific color – developing agent (such as o – toluidine) under the action of peroxidase, resulting in a color change. The depth of the color is proportional to the glucose content in the sample.
2. Reaction Process and Result Judgment
   • By comparing with a standard color – comparison card or using a special photoelectric color – comparison device, the absorbance value corresponding to the color depth of the sample after the reaction is measured to determine the glucose content in the sample. The detection of other physiological indicators in test kits is similar. Results are determined based on specific biochemical reactions and corresponding detection methods. For example, the absorbance of the reaction between creatinine in urine and picric acid is measured to determine the creatinine content.