We are usually talking about PCR, that is, when not antibodies are found, but the virus itself. At some point, they get a negative test result and rejoice. In fact, it's too early to rejoice.
In theory, one negative test would be enough to decide that a person is not contagious. In practice, this is not always the case.
To begin with, PCR is considered a very sensitive test. That is, if he found something, then this is it. And it seems that for the same reason, such tests were not particularly checked for false negative results.
When the test was invented, it worked perfectly, but only in the right hands and under ideal conditions. In practice, mistakes have gone. In this case, the test error was not calculated. It was believed that the test was super accurate.
In China, people with typical covid, fever, and cough were tested by PCR, and about one in three tested negative.
In Singapore, a lot of PCRs were specially done and it turned out that these tests confirmed the diagnosis only from the second, and sometimes even from the fourth time.
The study was even more interesting when they counted how many negative tests they get on some days of illness. It turned out that on the 1st - 3rd day of illness, about 10% of the tests were negative. On the sixth day of illness, more than 20% of the tests were negative. Well, by the 14th day - already 50%, because people there recovered.
It turns out that most often the virus was found in the early days. And before the first symptoms appeared, the virus was also most likely already isolated.
Now they propose not to let a person go to work until he passes two tests. The second test is suggested to be done within 24 - 48 hours. That is, rather every other day, but not the next day.
For the same reason, if a person fell ill with something similar to covid, and someone was already sick next to him, then even with a negative test he will be solemnly declared contagious.
Ask your doctor about the number of tests. Yesterday they said that someone broke the rules, the patient was released to work with one negative test, and he infected everyone. Don't let your doctor down!
