5. How to understand the sensitivity and specificity of the new crown antigen detection?
2022-03-21 10:16:09Source: China Youth Daily
Recently, a number of new crown antigen self-test products have been approved for marketing, which has attracted attention.
What is an antigen test, and what is the difference between it and the well-known nucleic acid test? Why add this new detection method? How is the detection accuracy? What does one need to do? Are there any caveats?Doubts about this new testing method ensued。
In an interview with a reporter from China Youth Daily and China Youth Daily, Qiu Hongling, a senior experimenter at the School of Public Health of Shanghai Jiao Tong University, explained,New crown antigen detection and nucleic acid detection are two different technical means.
First of all, the detection targets of these two detection methods are different. Antigen detection is generally the “coat” to detect the new coronavirus, and now most of them use a protein called “N protein” as the target antigen; nucleic acid detection is to detect the nucleic acid encapsulated in the virus, usually a small piece of the new coronavirus. Gene fragment.
In addition, nucleic acid detection can use instruments and reagents to amplify the nucleic acid obtained by millions or tens of millions, and even a small amount of virus can be detected; but the virus antigen cannot be amplified, The number of samples taken is the number of detection targets. Qiu Hongling believes that the accuracy and sensitivity of antigen detection are lower than nucleic acid detection, and nucleic acid detection is still the “gold standard” for new crown detection.
She also mentioned that antigen testing has its advantages, one of which is convenience, no need for machines, you can operate it at home, and you can get results in a short time; while nucleic acid testing involves multiple processing steps, about 4-6 Results in hours.
How accurate is the antigen test?
Li Jinming, deputy director of the National Health Commission’s Clinical Testing Center, said at a press conference held by the State Council’s joint prevention and control mechanism on March 15 on the situation of epidemic prevention and control and vaccination work that the sensitivity of the antigen detection reagents approved by our country At 75%-98%, the specificity was between 95%-99%.
How should sensitivity and specificity be understood?
Qiu Hongling told reporters that in general terms,Sensitivity corresponds to the probability of “not missing detection”,For example, she said, taking the nucleic acid test results as the reference standard, if the antigen test can detect 97 positive results out of 100 positive results, the sensitivity of this antigen test product is 97%; the specificity corresponds to “no misdiagnosis”. If 3 out of 100 positive results detected by antigen detection are false positives, then its specificity is 97%.
Since nucleic acid testing is so easy to use, why add antigen self-testing? Simply put, these two detection methods address different needs. An interviewee who is in quarantine at home in Shanghai told reporters that in the scenario of home quarantine, if there is an antigen self-testing product, she can screen it by herself first.
Qiu Hongling thinksThe main problem solved by antigen self-detection is to improve the ability of “early detection” of people at risk of infection.She explained that from the perspective of epidemic prevention, this is conducive to rapid screening of groups with high infection risk, which is convenient for social epidemic prevention and control; from the perspective of personal health, “early detection” is also conducive to early isolation and early treatment. Provide patients with access to medical resources as soon as possible.
Who needs to do antigen self-testing?
The “New Coronavirus Antigen Detection Application Plan (Trial)” issued on March 11 clearly pointed out the applicable groups: people who go to primary medical and health institutions for treatment, have respiratory symptoms, fever and other symptoms and have symptoms within 5 days; quarantine observation personnel, Including home isolation observation, close contact and sub-close contact, entry isolation observation, closed and controlled areas and personnel in control areas; community residents who need antigen self-testing. Qiu Hongling concluded that the applicable groups are those who think they are at a higher risk of infection.
What are the precautions for antigen self-testing?
It is not difficult to see from the instructions of the pre-sold antigen self-detection product that its use is similar to a home pregnancy test strip. Qiu Hongling emphasized that when self-testing and sampling, you should be a little “ruthless” with yourself, “because you need to scrape off part of the epithelial cells in the throat or nasal cavity to detect whether there is a virus.”
She also reminded that the testing supplies after the test should be properly disposed of. Whether it is self-testing for isolation observation or self-testing for community residents, it is necessary to pay attention to the fact that the testing supplies must be sealed as soon as possible. She said: “A positive antigen test corresponds to a time when an infected person is highly infectious. Therefore, once the antigen test is positive, for the health and social safety of yourself and your family, please do not move around at will, and immediately contact community managers or units to wait for further progress. processing.”