What is antigen testing? Who are the people who are suitable? Experts explain the COVID-19 self-test policy in detail

Beijing News reporter Wang Kara Editor Yue Qingxiu Zhang Xiulan proofread liu Baoqing Zhang Yanjun

According to the news released by the National Health Commission on March 11, the Comprehensive Group of the Joint Prevention and Control Mechanism of the State Council in response to the new crown virus pneumonia epidemic decided to add antigen testing as a supplement to nucleic acid testing. The plan clearly states that if community residents have self-testing needs, they can purchase antigen detection reagents for self-testing through retail pharmacies, online sales platforms and other channels.

What is antigen testing? Who are the people who are suitable? How effective is it? An industry insider in the field of genetic testing told the Beijing News reporter that the new crown virus antigen detection, which is often said to be home self-testing, has the characteristics of convenient operation and rapid results, but the sensitivity is lower than that of nucleic acid detection.

Beijing News: What are the applicable groups of people for the new crown antigen test?

A: According to the "Application Plan for Antigen Detection of the New Coronavirus (Trial)", the antigen testing is applicable to three categories, namely, those who go to primary medical and health institutions for treatment, accompanied by respiratory tract, fever and other symptoms and have symptoms within 5 days; isolation and observation personnel, including home isolation observation, close and sub-close contact, entry isolation observation, sealing and control areas and control areas; and community residents with antigen self-testing needs.

The "Plan" requires that if the antigen test is positive, regardless of whether there are respiratory tract, fever and other symptoms, residents should immediately report to their communities (villages and towns), and the community (villages and towns) will contact the emergency center to transfer residents to medical institutions that set up fever clinics for nucleic acid testing in accordance with the guidelines for the transfer of relevant personnel of the new crown pneumonia epidemic.

Beijing News: What are the testing methods for the new crown virus?

A: Nucleic acid testing is one of the most commonly exposed tests, but not the only one. At present, the detection methods of the new crown virus mainly include three kinds, namely nucleic acid detection, antibody detection and antigen detection. These three methods have their own characteristics in terms of sensitivity, detection speed, etc., and antibody detection is relatively special, mainly for those who want to determine whether they have been infected with the new crown virus and whether they have developed immunity.

After the nucleic acid detection and sampling is completed, the samples are sent to the laboratory for analysis, and the requirements for the laboratory environment and instruments are high. Compared to nucleic acid testing, antigen testing eliminates the need for laboratory PCR testing. Simply put, antigen detection eliminates the time of nucleic acid testing "running the machine".

Beijing News: What are the principles and characteristics of antigen detection?

A: There are four main structural proteins for covid-19: Spike protein (S protein), nucleocapsid protein (N protein), Membrane protein (M protein), and Envelope protein (E protein). Once the body is infected with the virus, these structural proteins can act as immunogens to stimulate plasma cells to produce specific antibodies. Most of the antigen detection products that have been approved use the N protein of the new crown virus as the target for antigen detection.

The sample source of antigen detection is usually nasal cavity, throat or saliva, etc., one of the characteristics of which is that the results can be produced quickly, about 15 minutes, which can realize the early shunting of the suspected population, the operation is relatively simple, and the price is also one of the cheaper of the three detection methods, but the sensitivity of antigen detection is also low.

Beijing News: Why is the sensitivity of antigen detection relatively low?

A: Due to methodological and self-principle limitations, the sensitivity of antigen detection is usually lower than that of nucleic acid detection, and the possibility of finding early infection is also lower than that of nucleic acid detection, and false negative results are prone to appear in testing. Usually, after infection with the new crown virus, antigen detection products can only be detected when the viral load in the infected person reaches a high level, that is, during the acute infection period.

When the viral load in people's bodies is lower than a certain lower limit, the sensitivity of antigen detection products will drop sharply and be difficult to detect, especially for asymptomatic infected people, it is easy to have false negative results. This is also the reason why the current antigen test cannot be used alone to diagnose the new crown virus infection, and it is still necessary to combine other diagnostic information such as nucleic acid detection and imaging.

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The new crown antigen home self-test product has not been approved in China,

Residents have to wait to buy

On the evening of March 11, a reporter from the Beijing News asked a number of pharmacies, all of which said that there were no new crown antigen testing products for sale. A number of pharmaceutical network platforms also said that they are waiting for further news, and the key is to see when such products are approved.

According to the Plan, the applicable population of antigen testing includes those who go to primary medical and health institutions for treatment, accompanied by respiratory tract, fever and other symptoms and have symptoms within 5 days; isolation and observation personnel, including home isolation observation, close contact and sub-close connection, entry isolation observation, sealing and control area and control area of personnel. Community residents who have self-test needs, after self-purchase, need to carefully read the instructions during self-testing, and standardize sampling, sampling, and result interpretation in accordance with the prescribed requirements and procedures.

The "Plan" also requires that if the antigen test is positive, regardless of whether there are respiratory tract, fever and other symptoms, residents should immediately report to the community (village and town), and the community (village) contact the emergency center to transfer residents to the medical institution that has set up fever clinics for nucleic acid testing in accordance with the guidelines for the transfer of personnel related to the new crown pneumonia epidemic. Asymptomatic residents with negative antigen tests can be closely observed and then undergo antigen testing or nucleic acid testing when necessary; symptomatic residents are recommended to go to a medical institution that has set up a fever clinic as soon as possible for nucleic acid testing; if it is inconvenient to seek medical treatment, they should self-isolate at home, avoid going out, and conduct antigen self-testing once a day for 5 consecutive days.

"The new crown antigen testing products are roughly divided into POC professional version and home self-test version. Up to now, a total of 5 new crown antigen rapid detection POC professional kit products have been approved in China, which need to be operated by professionals. The home self-test version of the product that can be purchased in pharmacies, online sales platforms and other channels has not yet been approved, and the introduction of new policies may be accelerated. On the evening of March 11, Liu Yi, chairman of Jiuan Medical, said in an interview with the Beijing News reporter that as a powerful supplementary means of nucleic acid detection, antigen detection products are convenient and fast. The liberalization of home-based self-testing will help early screening of the epidemic and reduce the risk of large-scale transmission. Especially in the face of national nucleic acid testing, the use of nucleic acid test (diagnosis) + home self-test (screening) may achieve 1+1>2 effect, and will also reduce the medical burden.

Duty Editor Gully