How did the nucleic acid test detect the new coronavirus?

Zhang Tiankan

In the fight against new coronary pneumonia, the detection of virus-infected persons through nucleic acid testing is already a method that everyone is familiar with

All living things except prions contain nucleic acid.

The nucleic acid detection of the new coronavirus is to detect some iconic gene fragments of the RNA genome of the virus, which can be detected with nucleic acid detection reagents

In the early stage of the new crown epidemic, Chinese researchers completed the analysis of the entire genome sequence of the new crown virus in a very short time, and through comparison with the genome sequence of other similar viruses, such as the coronavirus, found the specific nucleic acid sequence in the new crown virus

To detect the specific nucleic acid sequence of the new coronavirus, the new coronavirus nucleic acid (RNA) must be reverse transcribed into DNA, and then amplified or amplified by PCR (polymerase chain reaction) to detect the specific gene sequence

The role of PCR is to amplify DNA, that is, to select specific gene fragments of the new coronavirus as target DNA, and to amplify their sequences exponentially

The key to this nucleic acid detection technology is to amplify virus-specific DNA fragments

In 1953, Watson and Crick published a model of the DNA double helix structure, letting people know the molecular structure of DNA and ushering in the era of molecular understanding of life

In 1971, Professor Corana of the Massachusetts Institute of Technology and others put forward the idea of ​​nucleic acid amplification in vitro.

In 1976, Qian Jiayun, a Taiwanese scientist studying in the Department of Biology at the University of Cincinnati, Ohio, extracted high-temperature DNA polymerase from thermophilic bacteria found in the hot springs of Yellowstone Park, making the idea of ​​amplifying DNA a step forward

The shooter who invented PCR technology to complete the final shot was the American biochemist Kelly Mullis

After PCR was invented, some people praised that this invention divided biology into two eras: the pre-PCR era and the post-PCR era

The nucleic acid detection method is not only used for the diagnosis of new coronavirus infection, but also widely used for other viral infections

Why someone can be tested positive after multiple tests

Now, PCR is used in the diagnosis of new coronavirus infection, which is to confirm whether the subject is infected by the virus by rapidly amplifying specific gene fragments of the new coronavirus

Although the RT-PCR kit test is relatively reliable, there are false negatives at the beginning, that is, an infected person who has actually been invaded by the virus cannot be detected

The "New Coronavirus Laboratory Testing Technical Guidelines" promulgated by the National Health Commission of China stipulates that the types of specimens collected include upper respiratory tract specimens (pharyngeal swabs, nasal swabs, and nasopharyngeal aspirates) and lower respiratory tract specimens (deep coughing sputum).

Because the new coronavirus usually appears in the tissues and cells deep in the lungs, lower respiratory tract samples are the best for testing.
There are many viruses and the easiest to detect
.
However, deep lung tissue samples are not easy to collect, and when a patient coughs, some viruses can be carried to the upper respiratory tract, and the virus can also infect the upper respiratory tract, so upper respiratory tract specimens (pharyngeal swabs, nasal swabs, nasopharynx sampling) (Object) has become the standard sampling
.

The nucleic acid detection process includes multiple steps such as sample collection, sample processing, nucleic acid extraction, and PCR detection.
Now the entire average detection time takes 2-3 hours
.
Because it directly detects the viral nucleic acid in the collected specimens, it has strong specificity and relatively high sensitivity, so it is the main method of current new coronavirus infection detection
.

Further reading

Antibody testing and antigen testing are also diagnostic methods for new coronavirus infections

In addition to nucleic acid testing, another commonly used method is now used to diagnose new coronavirus infections, namely antibody testing
.
Under normal circumstances, after a person is infected by the new coronavirus, the immune system will produce antibodies to attack the virus.
Therefore, antibody detection is also a reliable way to judge whether a person is infected by the virus
.
However, since antibodies may not have been produced in the human body in the early stage of virus infection, and there is a window for detection, antibody detection can only be used as a supplementary diagnostic method for confirming the diagnosis or for tracing the source of a cluster of epidemics
.

Antibody detection includes colloidal gold method and magnetic particle chemiluminescence method.
The average detection time of colloidal gold method is about 15 minutes, and the magnetic particle chemiluminescence method generally takes 30-60 minutes
.

Taking the colloidal gold method as an example, the reagent contains viral antigens, and the presence of antibodies is detected by the antigen
.
After the body is infected with the new coronavirus, the specific protein of the virus stimulates the immune system to cause an antibody response
.
The colloidal gold method is to detect whether the human body has antibodies to the new coronavirus
.
If a person is infected by the new coronavirus and contains antibodies in the blood, it can bind to the antigen component of the new coronavirus on the reagent to form an antigen-antibody complex, which gathers a red reaction line at the test line, which proves to be positive (infected).
The opposite is negative
.

In addition, to diagnose whether they are infected by the new coronavirus, antigen detection reagents (kits) can also be used
.
The reagent paper or card is coated with new coronavirus antibody, and the presence of the antigen is detected by the antibody
.
The antigenic components of the new coronavirus mainly include N protein, E protein, and S protein.
When samples such as throat swabs, nasal swabs, sputum, serum, and plasma obtained by sampling are dripped into the kit, if these samples contain virus The antigen will form an antigen-antibody complex with the new coronavirus antibody on the reagent, and a red line will appear at the detection line, and the result will be quickly obtained
.
Now, the new crown antigen detection reagents produced by some domestic companies can quickly and accurately identify the Delta mutant strains, because the reagents (kits) contain the genetic sequence of the delta S protein specifically changed
.
However, antigen detection requires higher sensitivity.
Because the new coronavirus mainly invades the lower respiratory tract such as alveoli, sampling from the upper respiratory tract such as the nasopharynx and oropharynx may not necessarily collect the pathogen, or the number of viruses contained in the sample is small
.
Therefore, antigen testing, like antibody testing, is an auxiliary method for diagnosing new coronavirus infection
.

Must know

New crown virus infection and new crown pneumonia are not the same thing

It should be noted that the diagnosis of new coronavirus infection and the diagnosis of new coronary pneumonia are not the same thing.
The former is only a viral infection, and the latter is a symptom after a virus infection
.

At present, the gold standard for confirming new coronavirus infection is still nucleic acid testing
.
At the same time, antibody testing and antigen testing can be used as supplementary evidence
.

If you want to be diagnosed with new coronary pneumonia, you must add CT film evidence and clinical symptoms, such as fever, cough, and breathing difficulties, to diagnose new coronary pneumonia, and determine whether it is mild or severe, or asymptomatic infection , So as to take corresponding treatment and preventive measures
.