Max Mosterman needs a new haircut. To do this, he performs a rapid antigen test, which turns out to be negative. With the guide he goes to the hairdresser. Max had already predicted the negative result, after all, he had no signs of illness. But he did yesterday with Corona Injured. The day after tomorrow he will be with Birthmark To lie on the bed. But he still doesn’t know anything about it today.
At the time of the test, the viral load, that is, the amount of active viruses in his body, was still low. The polymerase chain reaction test that detects even the smallest amounts of viruses would have run that day and give a positive test result. The rapid antigen test did not reveal a low viral load. The next day, when viruses are multiplying in Max’s body, a quick test will also show a positive result.
The risk of infecting others in Max’s hairdresser is low (due to the low viral load) – but it is not zero. Max was probably more neglected that day because he received a negative result and is assumed to be in good health. Today, he doesn’t take it very seriously when it comes to washing hands and staying at a distance. And his mask sits relatively loosely when cutting his hair because it’s more comfortable.
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Well informed about the crisis with our Corona guide: Experts explain what you are up to now Covid-19 Need to know.
A negative antigen test result does not rule out SARS-CoV-2 infection
Because rapid tests are less sensitive, they only know the possibility of infection if there are many viruses in the sample. The test does not detect early infection with a low viral load. Virologist Christian Drosten also addressed this problem in his book NDR-Podcast Has already been indicated. There is a “detection gap” in the early stage of antigen testing and thus there is false safety during this period.
“Rapid tests may be less reliable than expected,” says Drosten. It is estimated that between 40 percent and 60 percent of injuries are ignored in rapid tests.
How important are express tests, then? Finally, in addition to the “detection gap,” there is a certain risk of tests being false positive or false negative. Manufacturers indicate sensitivity or specificity to this:
- The sensitivity It shows how often the test correctly detects the infection. The higher the sensitivity of the test, the more reliable it can detect a disease with a positive result.
- The Quality It indicates how likely it is that a healthy person will be recognized as such. Then the result is negative.
Manufacturers do not promise 100% security. The sensitivity of most tests is between 92.2 percent and 97.3 percent.
FOCUS Online Calculates
Assume the person with symptoms is contagious for ten days. Also, a patient who has no symptoms is contagious, but according to current knowledge for a shorter period, about seven days. Both are contagious three days before the first day on which symptoms appear, while the viral load in their bodies is still very low.
The rapid test will then run on seven out of ten days in the first case and four out of seven days in the second case. The Robert Koch Institute suspects that there are six times more people asymptomatic than asymptomatic.
To find out the chance of a rapid antigen test to detect an infection, the calculation is as follows: (7/10 * 1 + 4/7 * 6) / 7 = 0.59
This means that a rapid antigen test can detect infection in only 59 percent of cases – provided the test sensitivity is 100 percent. In the case of a test whose sensitivity is 95.4 percent, for example, at the end it is only that 56.3 percent of all cases. That’s just a little over half.
- Reading tip: A study by “Cochrane Collaboration” came to a similar conclusion: that antigen tests would only detect 58 percent of sufferers. Read more about studying here.
The quick tests still make sense
Online statistician Katarina Schuller still calls for the use of rapid antigen tests: they confirm benefit. Another guest on FOCUS Online, Because they reliably recognize infected people who pose a particularly high risk to others because they are carriers of many active viruses.
Schuller writes: “In the field of medical diagnostics, a high level of sensitivity is particularly important, which is why PCR tests are used there for the individual medical diagnosis of disease.” “In the non-medical field, it is more related to containing the epidemic then resuming social life. Therefore, factors such as speed of test result, ease of use and high availability are crucial factors. The diagnostic question ‘Do I have Corona?’ Can only be reliably answered with ‘Yes With the PCR test, Schuller continues, “The question is: Am I a danger to others?” It can also be answered “no” with a fairly high degree of certainty using a rapid test.
The statistician recommends quick testing by trained personnel if possible to avoid application errors. On the other hand, you have to be more careful with the self-tests recently conducted by some opponent – even if they are also antigen tests. On paper, they are practically as sensitive and specific as rapid tests in private centers or in the future also by doctors and pharmacists Perhaps by aid organizations. But if it is used incorrectly, there is a greater risk that it will produce incorrect results. “