In the meantime, you no longer necessarily have to go to a test center to be tested for the corona virus. Self-tests have been commercially available for a few days. Likewise, every citizen should be able to be tested once a week free of charge (e.g. in the pharmacy). Are the results of such rapid tests reliable?
Every citizen should be able to have one antigen test per week free of charge in pharmacies, medical practices or test centers. The federal government will bear the costs for this – however, the federal states have until the beginning of April to implement this measure as part of the national test strategy. You can also buy quick tests for self-use. There are the so-called self-test in pharmacies as well as in retail – e.g. B. in drugstores and even at discounters. In terms of their mode of action, they are almost identical to the usual rapid tests (= antigen tests). The advantage: You can do the tests at home. But how reliable can their results be?
How reliable are rapid tests and self-tests?
The Paul Ehrlich Institute has set minimum requirements for antigen tests: the specificity must be over 97 percent. This means that at least 97 out of 100 healthy people must be recognized as such. The sensitivity should be greater than 80, i.e. the test must detect at least 80 out of 100 infected people. According to the manufacturer, the specificity of all seven self-tests approved by the Federal Institute for Drugs and Medical Devices (BfArM) is at least 98 percent. False-positive results can therefore occur in some cases.
Antigen tests generally only have limited reliability
According to the Robert Koch Institute (RKI), with the right instructions, sampling by private individuals and the resulting results are comparable to sampling by medical staff. Studies should prove that.
However, application errors can severely limit the validity of the test. And even if done correctly, it is “only less likely” to be contagious if the result is negative. In addition, the informative value is limited in time, emphasizes the RKI. The result can be different the very next day. Therefore, a negative result is anything but reliable and by no means a license to disregard the Corona rules.
Important: PCR tests are always more reliable
It is known that rapid tests (= antigen tests) are in principle less reliable than the PCR tests taken into account for the official corona statistics. The two values sensitivity and specificity are important here: The specificity indicates how many non-infected people correctly receive a negative result. The sensitivity, in turn, indicates the proportion of those infected with the virus who actually correctly receive a positive test result.
Also interesting: The difference between PCR and antigen tests
According to the manufacturer and BfArM random samples, the sensitivity of the previously approved self-tests is a good 95 percent. The problem: The tests work best with a high viral load. Infected people with low viral loads—such as when the disease begins or is resolving—may go undetected.
Danger of false positive and negative tests
If one assumes that in a group of 10,000 people tested, 1000 actually carry the corona virus, then at least 50 of these infected people could falsely get a negative result in the self-test. They assume that they are not infected – and thus infect other people.
Of the 9,000 non-infected people in the example group, around 180 received a false positive result with a specificity of 98 percent. Until the result of the PCR test, they then assume that they are infected and can infect others. That can cause frustration.
To what extent the specified percentages are reliable under real conditions can hardly be foreseen at the moment. Only the experience of the coming months with rapid tests will show how big the problems caused by false positive and false negative results actually are – and also how well the rapid tests can keep the pandemic in check.
areas of application
Chancellor Angela Merkel, Federal Health Minister Jens Spahn (both CDU) and the governments in the federal states see laboratory-independent tests as an important element in their recently decided step-by-step strategy to soften the lockdown depending on the infection situation.
So far, rapid tests have already been carried out, for example, before entering retirement homes, in clinics and after cases of infection, for example in schools. Self-tests with a negative result could be sufficient in the medium term for a visit to the outside area of a restaurant. Provided that the regional corona situation would allow this.