Corona: Why you cannot rely on antigen tests

Corona: Why you cannot rely on antigen tests
Corona: Why you cannot rely on antigen tests

A coronavirus test apparently gives a clear result: positive or negative, infected or not. But no test is one hundred percent exact.

PCR testing is close, but expensive and takes a few hours to get the result. Antigen tests – that is, quick and self-tests – are faster and cheaper, but much less reliable. Anyone who believes that they are definitely not infected with a negative result may be deceptively safe.

Antigen tests are so reliable

The quick tests at a test site and the self-tests for at home work on the same principle; both are antigen tests. They strike when they find certain protein building blocks of the coronavirus in a sample from the nasopharynx. You can detect an acute infection within a few minutes.

Antigen tests are therefore much faster than PCR tests, but also significantly less sensitive. Both their specificity and their sensitivity are lower.

  • the sensitivity indicates how sensitive a test is. A low sensitivity test will miss infections. Then it gives negative results, which are wrong and should be positive.
  • the Specificity indicates how many healthy people a test actually recognizes as healthy. A test with low specificity will also work for other viruses or for other reasons. He then reports positive results, but they are false and should be negative.

Quick and self-tests are only reliable for certain infected people

Even if the sample is taken correctly and the test is otherwise carried out correctly according to the instructions: The reliability of antigen tests is only high in infected people who currently have a lot of viruses in their throat and throat.

The hit rate among infected people is highest in the first week after the onset of symptoms, according to a summary of 64 studies by the Cochrane network: On average, 72 percent of those infected with Covid 19 were correctly identified. In people without symptoms, however, the tests only gave a positive result in an average of 58 percent of those infected.

Many antigen tests are less sensitive

However, these are averaged values. In fact, not all antigen tests are as accurate as they should be. Around a fifth of the rapid antigen tests available in Europe did not have the sensitivity that the CE seal says they should have. This was the result of a study by the Paul Ehrlich Institute (PEI). Some antigen tests did not respond even at the highest viral load.

One List of antigen tests that did not meet CE requirements, has compiled the journal Laborpraxis. Most of the tests met the requirements. the antigen tests approved in Germany lists the Federal Office for Drugs and Medical Devices (BfArM).

So far, manufacturers have been allowed to certify their tests themselves in accordance with the current EU directive. An independent review is currently not required. From May 2022, however, according to information from the Paul Ehrlich Institute, this will change. Then an EU reference laboratory and a so-called notified body must be consulted. The tests are then examined in the laboratory and the data verified independently.

Antigen tests: false results before the first symptoms

However, even antigen tests with high sensitivity often incorrectly report negative results in infected people without symptoms. Coronavirus infected people can be very contagious shortly after infection if they have no or hardly any symptoms. Antigen tests then still often give a negative result.

Christian Drosten, head of virology at the Berlin Charité, points out: “In practical experience, the rapid antigen tests do not yet reliably become positive, especially at the beginning of the symptoms, where the patient is particularly infectious. And then they get positive, but you missed the important first days. You didn’t get that with PCR. “

Quick and self-tests in vaccinated people even more unreliable?

A negative result in an antigen test therefore does not reliably rule out that the person concerned is not infected after all. The antigen tests are apparently even more unreliable in vaccinated people, Drosten told “Bild”: “According to my preliminary assessment, it looks as if infections in vaccinated people cannot be detected so well by the rapid antigen test, especially in the first days of infection . Unfortunately, the study situation is not yet sufficient. “

Regardless of the reliability, the following also applies to every test: It is only a snapshot. Even if the result is correct, it can be different the next day. For example, if the infection only occurred a few days ago and the number of viruses in the respiratory tract is growing rapidly.

Conclusion

Antigen tests are not particularly reliable. But they can be a useful tool: When there are signs of a cold, they usually show correctly whether the sick person has Covid-19 or not. With continuous tests, outbreaks in groups such as school classes can also be detected and stopped early. Not every single test has to deliver a correct result here. It is sufficient if it is recognized at an early stage that the virus is circulating in the group.

However, a single negative test result is by no means a license to carelessly socialize and to take all precautionary measures. Keeping your distance, hygiene, ventilation and, above all, reducing your contacts, reduce the risk of infection and are still important after the test and vaccination as long as the pandemic is not over.

 
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