Recently, the new crown antigen detection is relatively hot, and I have seen many videos or articles on the Internet to introduce you how to do it or your own actual personal testing experience. But wait, is the COVID-19 antigen test really right for you?
What is the COVID-19 nucleic acid test?
I think everyone is already familiar with the new crown nucleic acid test.
But its disadvantage is that the process is complex, requiring special instruments and equipment to deal with, the entire inspection takes at least 4-6 hours to produce results, and the cost is relatively high.
What's different about antigen testing?
Antigen testing is to determine whether the test result is positive or not by using the antibodies in the kit to look for the presence of antigens in the specimen. In general, users can operate at home, generally 15 minutes to produce results, and the price is cheaper.
Sounds good, but why hasn't antigen testing been widely used in China before?
You're right, this detection method also has its own limitations: in a word, it is not accurate enough.
"False Negative" vs. "False Positive"
Before deciding whether antigen testing is really right for you, you need to understand two important concepts: "false negative" and "false positive", as well as the opposite of "sensitivity" and "specificity".
Among them, false negative refers to: the test result is negative, but in fact there is a case of infection with the virus, which is the so-called missed diagnosis, then the probability of an examination item not missing the diagnosis is its sensitivity. False positive means that the test result is positive, but there is no virus infection, which is the so-called misdiagnosis, and the probability of an examination item not misdiagnosing is its specificity.
"You're pregnant, congratulations!" "No, your big belly is just a stomach problem"
According to the standards of the nationally approved new crown antigen detection reagent, its sensitivity should reach 75%-98%, and the specificity should reach 95%-99%.
Let's use the concept of false negative and false positive just shared above to simulate it: suppose the sensitivity of the reagent you bought is 85%, the specificity is 97%, and you test it yourself at home. The result is negative, so what does that mean?
The overall epidemic prevention and control of our country is still very good, so the incidence of the natural population of the new crown is very low, about the average natural incidence of the whole population is less than one in a million.
Suppose you live in a city with a population of 10 million, and suppose that at some point in time the infection rate of the new crown virus is one in a million, the real number of new crown infections in the whole city should be only about 10 people (10 million x one in a million), with this antigen kit with a missed 15% rate of 15%, we will miss 1.5 of them (10x15%), but because the denominator is large enough, the probability of a negative result is still close to 100%.
Rough calculation formula: more than 9 million true negative results / true negative results + 1.5 cases of missed diagnosis x 100%*.
Do antigen kits really work?
Seeing this, you may think, antigen kits are not very good?
Don't worry, in fact, the real problem is when you test for a positive result.
Using the assumption above, using this antigen kit with a misdiagnosis rate of 3% for screening the general population, we will get about 300,000 positive results (10 million x 3%), but the number of patients who are actually infected with COVID-19 and detected is only 8.5 (10-1.5).
At this point, the probability of your own positive testing is only about 8.5/300,000, or less than 3/100,000.
In other words, unless the incidence of new crown in the tested population is already very high, the positive results obtained by random antigen testing in the vast sea of people are basically false positives, that is, misdiagnosed.
Image source: Weibo
In this case, what if you do an antigen test at home and the result is positive? Although I know that you are most likely wronged, in order to take off your hat and prove your innocence, you still have to go to a nucleic acid test for a second confirmation.
So if this is the case, why not do nucleic acid testing directly at the beginning? Well, good question!
Applicability of antigen testing
Therefore, looking back, when the National Health Commission released the "Application Plan for Antigen Detection of The New Coronavirus (Trial)" in early March, it actually pointed out its use scenarios very clearly:
Persons who go to primary medical and health institutions for treatment, accompanied by respiratory tract, fever and other symptoms and have symptoms within 5 days;
Quarantine and observation personnel, including personnel in home isolation observation, close and sub-close contact, entry isolation observation, sealing and control area and control area;
Residents of communities with antigen self-testing needs.
The first two of them are people with a relatively high risk of infection, in other words, the incidence of new crown in this part of the population will be significantly higher than that of the general public, so it is of practical significance to use antigen testing to help quickly determine.
Although the third party has left a mouth for the general public with self-needs, the Health Commission also calls on everyone to "do antigen testing at will in the general population" at will. The reason is that the general population has done antigen test results even if it is positive, generally it is also false positive, you have to do nucleic acid to "prove self-innocence", so it is in vain to increase the annoyance, redundant.
Note: The above calculation model is approximately simplified
Source: Zhuo Zhengpu Health Express
Edited by: Yeah Reviewer: Xiao Ran