In view of the highly infectious Omicron variant, the uncertainty increases. Are the widely used antigen tests even reliable? What is known about their effectiveness.
The numerous swabs from the mouth and nose provide some certainty in the corona situation every day. Due to the rapidly increasing number of cases due to the advancing Omikron variant, the test strategy in Germany is once again moving into focus.
The federal government wants to enable early free testing from quarantine not only with PCR tests, but also with “high quality” antigen tests. However, many experts consider this idea to be a bad one and point to the limitations of antigen testing. But the PCR test capacities are not inexhaustible either.
Findings so far show that antigen tests, which include rapid and self-tests, often fail, particularly when the viral load is low. “A free test with only an antigen test, that is not possible,” says the chairman of the professional association of German laboratory doctors, Andreas Bobrowski, of the German press agency. He considers such freedom to be a problematic signal and objects: “After a few days, the tests would almost all be negative and you would show people a false sense of security. You have to be very cautious. “
Different test principle – different reliability
The fact that PCR and antigen tests have different levels of accuracy is due to the way they work. In PCR tests, the genetic material of the pathogen is detected in complex work steps with the help of a special PCR device, explains Martin Roskos from the laboratory service provider Synlab Germany. In antigen tests, proteins that are characteristic of the virus are detected – using self-tests or rapid tests, for example at home or at a test center. A laboratory-based evaluation of samples, which is generally more precise than self-tests or quick tests, according to Roskos, is also possible.
The difference in reliability is, however, especially in the case of asymptomatic corona infections in all antigen tests compared to the PCR test, “quite considerable”, says the Lübeck laboratory doctor Bobrowski. “The main problem that both the laboratory-based antigen test and the rapid test have is simply the significantly lower sensitivity when the viral load drops.” This also applies to the Omikron virus variant.
The sensitivity is one of two values that play a role in the reliability of corona tests. While the specificity indicates how many non-infected people correctly receive a negative result, the sensitivity indicates the proportion of those infected with the virus who actually receive a correctly positive test result. So what can be deceptive: A negative antigen test result does not necessarily rule out an infection – and can also result from a low viral load at the time of the test.
Differences in sensitivity and specificity
Specifically, both the sensitivity and the specificity of the coronavirus in PCR tests are almost 100 percent – even with a low viral load, according to the experts. In those infected with symptoms, the antigen tests worked quite reliably, says Bobrowski: the sensitivity is around 80 and the specificity around 95 percent. In the case of asymptomatic courses with a low viral load, it is different: Here, the antigen test only turns out correctly positive in about half of the infected. “That means we’re overlooking half of it.”
Initial study results also indicate that antigen tests in Omikron could miss early infections. According to a study by a team led by Blythe Adamson (University of Washington), a direct daily comparison of PCR tests in saliva and nasal antigen tests in a cohort of 30 people showed that the latter often only recognized an omicron infection much later. Most Omikron infected people were therefore infectious for a few days before this could be detected by rapid antigen tests. The study has not yet been published in a specialist journal.
Omikron recognizes the majority of rapid tests
The Paul Ehrlich Institute (PEI) points out in an online overview that antigen tests were not developed to reliably diagnose corona infection, but to identify people with a very high viral load quickly and easily. The PEI also states: The tests could only detect an infection, even with the Omikron variant, if there was a high viral load at the time of the test.
But: In principle, the majority of the corona rapid tests offered in Germany can detect the Omikron variant, according to PEI. The president of the institute, Klaus Cichutek, last said in the ZDF “Morgenmagazin” that the institute has now rated over 250 test products at a higher level of sensitivity and at least 80 percent achieved this level.
Health Minister Karl Lauterbach (SPD) had announced a “positive list” for rapid tests that Omikron can easily detect. A spokesman for the Ministry of Health explained that the data given by the PEI is an interim status. The creation of the complete list continues. He affirmed that rapid tests do not offer 100 percent certainty, but ensure more safety in everyday life.
PCR capacities at the limit?
Most recently, the Marburger Bund doctors’ association warned of possible bottlenecks in PCR tests. The Ministry of Health stated that the possible weekly capacity of 2.4 million tests would not yet be exhausted with currently up to 1.5 million PCR tests. The chairman of the Association of Accredited Laboratories in Medicine (ALM), Michael Müller, said: “The burden in the laboratories is considerable, but I see no reason for too much concern.” With increasing test activity and limited capacities, it is important to focus more on the national test strategy.
Should the number of cases increase so massively that capacities would become scarce, from the perspective of laboratory doctor Bobrowski, PCR tests by people from risk groups and by employees of the critical infrastructure would have to be evaluated in order to obtain results within 24 hours. The national test strategy provides for such a prioritization.