Recently, the "New Coronavirus Antigen Detection Application Program (Trial)" was released, and the antigen detection kit was also put into use simultaneously.
The reporter combed and found that as of March 16, the State Food and Drug Administration had approved 12 new coronavirus antigen detection reagent products, 9 of which were based on the colloidal gold method.
What is colloidal gold method?
A reporter from Science and Technology Weekly invited Associate Professor Yan Qin from the Department of Pathogen Biology, School of Basic Medicine, Nanjing Medical University to conduct popular science.
"To answer this question, we must first know what the new coronavirus looks like." Yan Qin introduced that the new coronavirus is a simple life form that contains nucleic acid and is covered with an envelope. The envelope also marks the "identity" of the virus. protein.
When the body finds the "enemy", that is, after recognizing the protein of the virus, it will initiate a series of immune responses to produce antibodies.
The new coronavirus antigen test is actually the detection of viral proteins.
Colloidal gold is a colloidal form of gold-labeled biological macromolecules. In the process of detecting new coronavirus antigens, it can be used as a color medium to combine with antibodies, which is equivalent to staining the antibodies. Less than one antibody can be observed.
The basic principle of the colloidal gold method new coronavirus antigen detection kit is actually "antigen-antibody reaction".
Antigens and antibodies are like a "key and lock" relationship. When the body's immune system recognizes the virus antigen, it produces antibodies, and the antibody further specifically binds to the virus antigen, and this specific binding can also occur in vitro. The development of antigen detection kits increases "feasibility".
The colloidal gold test paper is only about the length of the palm of your hand, and it is also marked with "C" and "T". What does this mean?
In fact, this little test paper can be very knowledgeable.
Yan Qin told reporters that three different antibodies were immobilized in different sections of the test strip.
Gold-labeled antibodies are colloidal gold-labeled antibodies that can recognize a certain antigen of the new coronavirus and are located on the colloidal gold binding pad between the sample well and the detection line (Test line, T line for short).
The T-line antibody is located on the detection line. This antibody recognizes the same antigen as the gold-labeled antibody, but in a different location.
This means that the antigen of the new coronavirus can be recognized by the gold-labeled antibody and the T-line antibody at the same time, which is equivalent to "double insurance", which greatly improves the specificity of the detection.
The third antibody is the C-line antibody, which is located on the control line (Control line, C line for short) and can recognize the binding gold-labeled antibody.
These three antibodies are like "triple checkpoints", always ready to "intercept" antigens.
During detection, after the sample is placed in the sample well, the sample moves forward along the nitrocellulose membrane due to capillary action.
If the sample contains the new coronavirus antigen, it will first be recognized and bound by the gold-labeled antibody on the binding pad to form a "virus antigen-gold-labeled antibody" complex, and continue to flow in the direction of the absorbent pad at the end of the test paper. It is captured by the T-line antibody again and forms a "colloidal gold antibody-virus antigen-T-line antibody" complex on the T-line.
The T line appears red due to the massive aggregation of colloidal gold.
Since the amount of gold-labeled antibody in the test strip is surplus, it will not be all trapped in the T line, and will continue to flow toward the absorbent pad.
When it flows to the C-line, it is captured by the C-line antibody that specifically recognizes the gold-labeled antibody, and forms a "gold-labeled antibody-C-line antibody" complex on the C-line.
Also due to the large accumulation of colloidal gold, the C line appears red.
"Because the C-line antibody has a very strong ability to recognize gold-labeled antibodies, as long as the reagent is qualified, the C-line will definitely appear red." Yan Qin said that the appearance of the C-line means the detection process is successful and the test results are credible.
At this point, it is the T line that needs attention.
The appearance of a red T line indicates that the virus antigen has been successfully "captured", which means that the new coronavirus antigen test is positive.
She reminded that it should be noted that no matter whether the T line is colored or not, as long as the C line is not colored, the test result is invalid, and a new test strip needs to be used for retesting or nucleic acid testing.
Immunocolloidal gold technology is not a new product, it has been very mature since the 1970s.
For example, the common early pregnancy test strips in life use this technology to determine whether pregnancy is by detecting human chorionic gonadotropin in urine.
On the other hand, antigen detection has a wide range of applications.
"It can be said that the antigen-antibody reaction is the basis of immunological detection technology." Yan Qin said, for example, the most commonly used two-and-a-half test indicators for the diagnosis of hepatitis B include hepatitis B virus surface antigen, as well as the familiar essence of ABO blood type identification. It is also a detection method based on the difference of red blood cell surface antigens.
Previously, we saw that many experts mentioned when interpreting antigen testing that this method cannot replace nucleic acid testing. Why?
Yan Qin explained that the sensitivity of antigen detection reagents that have been approved in my country is about 75%-98%, which is lower than that of nucleic acid detection.
The reason behind this is that, compared with nucleic acid detection, antigen detection has one less important step - amplification.
"During nucleic acid detection, viral nucleic acid is used as a 'template' for continuous replication, and even a small amount of viral nucleic acid can be 'captured' through amplification." Therefore, during antigen detection, if the virus content in the sample is not high or the sampling is unsuccessful , may lead to missed diagnosis.