If a positive result is found in the nucleic acid test, can it be determined that COVID-19 is infected? Actually, not necessarily. We can use mathematics to explain this truth.
Nucleic acid detection cannot guarantee complete accuracy, that is to say, there may be false positives and false negatives.
Nucleic acid test results:
There are two concepts to evaluate the accuracy of nucleic acid detection:
If the sensitivity and specificity are both 100%, it is completely accurate, but in reality, due to various reasons, it is often impossible to achieve the accuracy of 100%, and at most it can only be close to 100%.
Suppose a country, after a round of investigation, has 1‰ people infected with COVID-19. Now after a round of nucleic acid detection, the sensitivity and specificity of this nucleic acid detection is 99%. Now Zhang San has done this nucleic acid test with 99% sensitivity and specificity, and the result proves that Zhang San's first test result is positive. What is the probability of Zhang Sanzhen's infection?
Let's analyze it. At present, there are 1‰ infected people in this country. Now Zhang San has two possibilities. The first one is infected, and the second one is not. The infection rate may be 65,438+0 ‰, and the uninfected rate may be 999‰, but whether he is infected or not, he may test positive or negative. Then after the first nucleic acid test, Zhang San's results are as follows.
At present, Zhang San is known to be positive, so we don't have to worry about the negative of the second line in the table. He can only be really infected and tested positive, or not infected and tested false positive, that is, there is only the probability of the first row in the table, so the following formula can be obtained.
The final calculation result is about 9%, that is to say, if a kit with sensitivity and specificity of 99% is used to detect Zhang San, and the test result is positive, then only 9% of him may be really infected, and most of him may not be infected, which is likely to be a false positive. The reason is that the infection rate in this country is very low, only 1‰, far below 1%.
At the time of the second test, Zhang San's infection rate was not 1‰, because he was positive after the first test, so his infection rate has changed from 1‰ to 9%, and the following table can be obtained.
Then if the second test is positive, calculate the probability of true infection.
Therefore, if the test is positive twice, there is a 9 1% probability of being confirmed as an infected person. If you are still not at ease, you can do it three times, four times and so on. Therefore, if you want to diagnose a person, it is not accurate to rely solely on nucleic acid detection. Conversely, it is not accurate to exclude a person only by nucleic acid testing, which means why we should check blood, antibodies, lung CT and so on when we are in hospital.