The principle of rapid detection of new coronavirus
Time: Jan 05, 2022

One is similar to the colloidal gold test paper commonly used in influenza virus testing.

Colloidal gold test paper, in fact, is to use a colloidal form of gold to label biomacromolecules, and fix it on the PVC test paper, leave a sample hole, set a detection line and a quality control line, so that the sample is added to the sample hole After that, add a few drops of liquid medium and let the liquid flow to the absorbent pad on the other end of the test paper. Chromatography is performed on the test paper, usually for 10 to 15 minutes. After the immune reaction on the test paper occurs, you can see the colloidal gold aggregation. Is there a red band at the position that is visible to the naked eye, so you know the test result.
At present, there are two types of colloidal gold test strips that have been publicized and reported on the new coronavirus. One is to detect the antigen of the new coronavirus.

This method uses the double antibody sandwich method, that is, for the same antigen, there are two antibodies on the test paper, but in different positions on the test paper. The colloidal gold-labeled antibody is one of the antibodies against the antigen, on the binding pad of the test paper.

For example, throat swabs, nasal swabs, and alveolar lavage fluid samples are dripped into the sample hole, and then a few drops of flowing medium are dripped. If there is a specific virus antigen in the clinical sample, it will be bound to the gold on the pad. Recognized by the target antibody, an antigen-antibody complex is formed. At this time, continue to flow in the direction of the absorbent pad. On the detection line, there is another antibody against the antigen. The epitope recognized by this antibody is different from the gold-labeled antibody, so the same antigen can be simultaneously affected by these two antibodies. Identified. At this time, a part of the gold-labeled antibody will be trapped at the detection line, so that the color is developed on the detection line.

Of course, the amount of gold-labeled antibody is definitely surplus, so it will not only be intercepted at the detection line, but will continue to flow in the direction of the absorbent pad. At this time, there is a quality control line. On the line, there are antibodies specifically directed against gold-labeled antibodies (that is, anti-antibody, which is essentially an antibody, but the antigen object it recognizes is also an antibody). Because the quality control line has a very strong ability to recognize gold-labeled antibodies, this line will definitely show color. This is also the principle of quality control. If this line does not show color, then the test result is invalid.

Therefore, the judgment of negative and positive is very simple, both lines are colored-positive; only the quality control line is colored-negative; other than these two cases are invalid experiments.

JINYE can provide various types of COVID-19 antigen rapid Test Kits, which can complete SARS-CoV-2 antigen self-test at home and get results quickly, and there are also professional versions in hospitals. And these new crown reagents have FDA and CE certificates.Colloidal gold covid antigen test can be used for early screening of infection and is an effective supplement to nucleicacid (PCR)detection.

The above is the double antibody sandwich method for detecting antigens, and the other is to detect IgM antibodies specific to the new coronavirus in the patient's blood. This is the colloidal gold test paper for the IgM capture method.

First of all, we must understand the law of human antibody response——

When the human body is exposed to foreign antigens, the earliest antibody produced is IgM (Ig is the meaning of Immunoglobulin/immunoglobulin), but it also needs to be about one week after the body is exposed to pathogen stimulation, so in terms of the detection time window, it is not as good as nucleic acid. The test comes quickly.

IgM is actually the secretion form of B cell receptor (BCR) on the surface of B lymphocytes. This is a patrol prepared in advance during the development and maturity of the human body, but it is equivalent to a recruit, and has not really seen the enemy. After IgM recognizes the antigen, the B cells that produce these IgM will enter the lymph node, where it is trained with the help of the antigen-presenting cells APC (mainly dendritic cell DC) and helper T lymphocytes that have been analyzed for hostility. The enemy’s qualitative leap, the B cells themselves have enlightened and evolved, which is equivalent to the level 1 fighter in Civ becoming a level 6 fighter. The result of evolution is the differentiation from B cells to plasma cells, which can be secreted in large quantities. Antibodies, and the affinity of these antibodies is much higher than that of IgM. This is IgG. IgG will exist in the body for a long time, and some may even exist for decades. However, IgM only appears when the pathogen first arrives and lasts for about a week. Therefore, IgM can be used to reflect whether the body is in an acute infection state.

After understanding the body's antibody response rules, we know that if IgM antibodies against the new coronavirus can be detected in human blood, then clinical diagnosis can be made.

The principle of the colloidal gold test paper of the IgM capture method is well understood.

The layout of the test paper is still the same, but everything is different in each place——

The sample pad is dripped with blood samples (serum), which may contain IgM antibodies against the new coronavirus, and the gold-labeled antigen reserved at the binding pad, rather than the gold-labeled antibody. This antigen is a protein of the new coronavirus (recombinantly expressed in vitro). In this way, IgM will bind to this antigen and carry the colloidal gold. The position of the detection line is the antibody against human IgM, so that if there is IgM in the blood sample, a part of it will be trapped by the antibody in this place, but if these IgM cannot recognize the gold-labeled coronavirus antigen at the binding pad, then the test The thread will not develop color. So this is the principle of the detection line.

For the quality control line, here is the laboratory-prepared antibody against the gold-labeled antigen at the binding pad. Therefore, regardless of whether there is specific IgM in the sample, as long as the flow medium is added, the gold-labeled at the binding pad The antigen will be captured by this place, that is, the quality control line will definitely be colored. Then this quality control is as simple as the above, about 15 minutes, it can be judged based on the color rendering of the two lines.

After the introduction of the principle, the biggest advantage of colloidal gold is that it is fast and convenient.

But the shortcomings are also obvious-the throughput is not high, and the accuracy of the test paper is highly dependent on the specificity of the antibody. If the quality of the antibody is not good, it is easy to cross-react and cause misjudgment. In addition, the time of IgM production is later than the time of virus replication, so the detection window is not as good as nucleic acid detection.
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