Countless daily nasal and oral swabs provide some certainty as to whether or not someone is infected. Due to the rapid spread of the Omicron variant, the German government is putting the weight of its strategy on the tests, because it wants to allow the early quarantine to be stopped not only by PCR but also by “high quality” antigen (rapid). However, many experts do not find the idea good and cite the limits of these tests. On the other hand the safest molecular PCR test is not in abundance. Knowledge and experience so far show that especially when the viral load is low, usually the antigen tests, to which rapid and selftest belong, often fail. “Free antigen tests are not enough,” said Andreas Bobrovsky, president of the German Professional Association of Laboratory Physicians at the DPA news agency. Bobrowski believes that such free tests “emit” a problematic signal and insists that “after a few days everything is negative and gives people the wrong security. One must be very wary of them.”
Different test logic, different reliability
The fact that the security levels are different between the molecular PCR test and the antigen test is due to their different mode of operation. In PCR, the genetic material of the virus is detected in complex processing steps with the help of a special PCR device, as described by Martin Roscos from the Synlab laboratory in Germany. Antigen tests (rapid or self test) detect proteins that are characteristic of the virus, whether they are done at home or in special diagnostic centers. There is also the possibility of diagnosing these tests in the laboratory, which is generally more accurate. The difference in reliability lies especially in the asymptomatic. The safety of PCR tests from those antigens is “significantly higher,” Bobrowski believes. “The main problem with both the laboratory antigen test and the rapid test is their significantly reduced sensitivity as the viral load decreases. And this is also the case with Omicron.”
Sensitivität is one of the two parameters that play a role in the reliability of coronavirus tests. While Spezifität shows how many uninfected people rightly get a negative result, sensitivity indicates the percentage of people infected with the virus who got a really good positive result. But what is it that can deceive? A negative antigen test does not necessarily rule out the possibility of infection, which may be due to the fact that the viral load was low at the time of the test. These two parameters in molecular tests fall within almost 100%, even at low viral load. “In infected people who show symptoms, antigen tests work almost safely,” says Bobrovsky. “Sensitivity reaches 80% and specificity almost 95%. But in asymptomatic with a low viral load, 50% of those infected look positive. In other words, we lose the other half.” Even the researchers’ initial studies show that antigen tests on Omicron in the early days of infection may not detect it.
The majority of tests “recognize” Omicron
A team study around Blitz Adamson of the University of Washington showed that in a direct daily comparison of molecular tests in saliva and nasal smears in a group of 30 people, the latter were later diagnosed with Omicron infection. Most already had the infection within a few days before rapid tests were able to diagnose the virus. The study has not yet been published in a scientific journal. The Paul Ehrlich Institute points to an online study that points out that antigen tests were not developed to provide a safe diagnosis, but to quickly and easily identify those infected with a very high viral load. But for its part, the Institute admits that these tests detect infection, even with the Omicron variant, since when they are carried out the viral load is very high.
In general, however, most of the rapid coronavirus tests available in Germany can identify Omicron. 250 types of tests available in Germany have a high sensitivity index and at least 80% achieve good results. Health Minister Carl Lauterbach has announced the publication of a so-called “positive list” of rapid tests that can diagnose Omicron. A ministry spokesman said the Paul Ehrlich Institute’s data was temporary and would last until a full list was drawn up, noting that rapid tests did not offer 100% security, but only some security in everyday life.
DW – Josephine Kaukemiler / Irini Anastassopoulou
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