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Since the breakout in late 2019, COVID-19 has repeatedly failed to be fully contained. At present, the Omicron strain has replaced the Delta strain as the main circulating strain in the world. Omicron variant strains are characterized by strong infectivity and more complex and hidden transmission. It is therefore difficult to control the outbreak of the Omicron variant strains.
In order to further optimize the SARS-CoV-2 testing strategy, the surveillance mode of "antigen screening and nucleic acid diagnosis" has been adopted by some countries in accordance with the needs of epidemic prevention and control, that is, supplementing nucleic acid testing with antigen testing.
There are several methods for testing for SARS-CoV-2, and the best method depends on the specific situation. Here are some common methods:
Nucleic acid testing
The nucleic acid test is to amplify specific segments of viral nucleic acid through polymerase chain reaction (PCR) to determine whether the virus exists. So nucleic acid test is also called the PCR test.
Samples are first collected from the throat or nasal swabs. SARS-CoV-2 RNA is extracted from collecting samples and is reversed to DNA through reverse transcriptase. The specific fragments of viral nucleic acid are continuously amplified by PCR reaction after adding the SARS-CoV-2 specific upstream and downstream primers designed according to the relevant sequences.
If the target sequence of the SARS-CoV-2 exists in the reaction system, the DNA will be amplified and fluorescence will be emitted. Each time a strand of DNA is amplified, a fluorescent molecule is produced. Finally, according to the fluorescence quantitative analysis of DNA concentration, to confirm whether infect SARS-CoV-2 and determine the concentration of SARS-CoV-2. Therefore, nucleic acid detection has high sensitivity and specificity.
Nucleic acid testing of SARS-CoV-2 is crucial for early clinical detection, early reporting, early isolation, and early treatment. It is a key technical support for effective prevention and control of the SARS-CoV-2 outbreak, and also the current "gold standard" of SARS-CoV-2 detection.
Antigen testing
An antigen test is used to identify proteins on the surface of the SARS-CoV-2. It can be performed quickly and produces results in a matter of hours. However, it is not as sensitive as a PCR test and may miss some cases of COVID-19.
Currently approved tests for SARS-CoV-2 antigen are mainly for detecting viral nucleoprotein (N). The SARS-CoV-2 N protein first binds to the gold-labeled antibody on the sample mat, moves forward under the action of capillary, reaches the test (T) line, and is captured by T-line antibody, forming the "gold-labeled antibody-SARS-CoV-2 N antigen-T-line antibody" complex. As colloidal gold accumulates in large amounts at the T-line, a red line is created, which is judged as positive. If not, the result is negative.
The surplus of gold-labeled antibodies will continue to flow to the quality control (C)-line, where it is captured by C-line antibody, forming a "gold label antibody-C-line antibody" complex, which produces a red line due to the action of colloidal gold. The formation of the C-line is used to determine whether the experiment is effective or not. Improper operation steps leading to chromatographic failure or kit invalid problems will cause the C-line to fail to color. When there is no strip on the C-line, no matter whether there is a strip on the T-line, the result is invalid.
It is worth noting that the addition of antigen testing as a supplement to nucleic acid testing is a move to further optimize the SARS-CoV-2 testing strategy, which is conducive to improving "early detection" capacity in areas with community transmission. However, due to the limitations of its methodology, the results of the SARS-CoV-2 antigen test cannot replace the results of the nucleic acid test, which is still the basis for the diagnosis of SARS-CoV-2 infection.
Antibody testing
An antibody test is used to detect antibodies in the blood that are produced in response to a SARS-CoV-2 infection. It is typically done a few weeks after the onset of symptoms and is used to determine if someone has had a previous infection.
After SARS-CoV-2 enters the human body, it takes an average of about five days for the incubation period. Most people will produce antibodies in the body two days after they start showing symptoms. The human body produces two types of antibodies, IgM and IgG. The dual antibody test is suitable for people who have negative nucleic acid tests but specific clinical symptoms.
IgM antibody production begins a week after infection and peaks on days 10 to 21, lasting almost five weeks. The presence of IgM antibodies indicates a recent infection and can be used for early diagnosis of infection.
IgG antibody is a long-term protective antibody. It begins to produce 2 weeks after infection with the virus and reaches its peak on the 25th to 30th day. It lasts for a long time, and can even last for more than 6 months.
Antibodies appear faster and more frequently in people who have been vaccinated against SARS-CoV-2. Because the vaccine leaves an immune memory in your body, when you're infected with the real virus again, your body will rely on that immune memory to produce more antibodies in a shorter time.
In general, the PCR test is considered the most reliable method for diagnosing SARS-CoV-2. However, other methods may be used in specific situations, such as when a rapid diagnosis is needed or when PCR testing is not available. It's important to note that no test is perfect, and a negative test result does not necessarily mean that a person does not infect with SARS-CoV-2.
In comparison, antigenic positive development is slightly later than nucleic acid positive development, while the antigenic negative transition is earlier than the nucleic acid negative transition. Antigen sensitivity is lower than nucleic acid detection sensitivity. Both methods have the possibility of false positives and false negatives. Multiple days of continuous antigen testing can improve accuracy. Antigen detection is simple and fast, and results can be obtained within 15-30 minutes, which is suitable for rapid self-test and screening at home. Nucleic acid testing requires sampling by professional personnel and testing by professional organizations.
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