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Corona test: what does the Ct value reveal about the risk of infection?

Not every person infected with corona inevitably infects others. Experts have been discussing why this is the case for months. The virus concentration could play a decisive role - and this is indicated by the Ct value.

The PCR test is considered the standard procedure in the corona pandemic. It is used nationwide to test people for the corona virus. Its big advantage: it is very sensitive. This means that if the sample is taken correctly, it will recognize almost anyone infected.

But at the same time, the high sensitivity could also lead to the test showing a positive result, even though the corona infection has progressed so far that hardly any viruses exist in the body of the person affected. The PCR test not only detects living virus particles, but also virus RNA (genetic material). And if the virus concentration is low, the risk of infection is also low. In short: Those who test positive for Corona do not necessarily have to be infectious.

Experts like the virologist Christian Drosten suggest using a certain characteristic value of the PCR test to estimate the infectiousness of an infected person: the Ct value. However, according to the current state of knowledge, the measure is only suitable to a limited extent.

This is how PCR tests work:
The PCR method uses the genetic material of the virus, the RNA, as a basis. For this purpose, genetic material is taken from a test person, which is usually obtained by taking a swab in the mouth, nose or throat. If someone is infected with SARS-CoV-2, genetic material from the virus can be found in the sample. These RNA molecules are isolated in a chemical process and replicated using a so-called polymerase chain reaction.

What does the Ct value mean?

The Ct value ("cycle threshold") can give an indication of the amount of virus that a patient is carrying. It is collected by the laboratory and shows how many rounds the PCR method must be used before the virus can be detected. The fewer viruses there are, the more cycles are needed until the genetic material is so enriched that it can be used as virus detection.

  • If the Ct value is low, it means there is a high viral load in the sample.
  • If the Ct value is high, this means that the virus concentration in the sample being examined is usually lower.

In principle, the Ct value can be used to read how high the corona virus load was in the throat from which the sample came. According to laboratory doctors, Ct values ​​above 30 indicate a low virus concentration and Ct values ​​above 35 indicate a very low virus concentration. With these values, a patient is very likely no longer contagious. According to the Robert Koch Institute, no virus can be reproduced from samples with a Ct value of more than 30 in laboratory tests.

Laboratory results: The Ct value determined in the PCR test can be used as a measure of the actual amount of virus RNA present. (Source: IP3press / imago images)

How meaningful the Ct value actually is

But there are increasing voices that the Ct value is only partially meaningful, because a high Ct value does not always mean that the patient has a low viral load in the throat. Because the measured virus concentration is also dependent on the technique of sampling, explains Professor Ortwin Adams from the Institute of Virology at the University Hospital Düsseldorf to the science magazine "Spektrum". "If the smear is bad, you immediately have a lower viral load in the sample and thus a higher Ct value," says Adams.

Medical laboratories also warn against misinterpreting Ct values ​​in the PCR test. The MVZ Labor in Ravensburg writes in a message for doctors that in addition to "non-optimal sampling", subsiding or weak infections can also be the cause of high Ct values. Ultimately, even in seriously ill patients, the virus may have already migrated from the throat area to the lungs, which is why the virus concentration is low.

Ct values ​​have not yet been taken into account

According to information from the "Süddeutsche Zeitung", the health authorities are usually not informed about the Ct value of infected people. Only in individual cases do the authorities know from the laboratory results how infectious a person is who has tested positive.

The Robert Koch Institute has only recommended using the Ct value for assessment in one case: Patients with a severe course may be released from isolation ten days after the onset of symptoms if they have been symptom-free for 48 hours and their Ct value is higher than 30 is.

According to the RKI, patients with mild symptoms or patients with no symptoms at all should not be "de-isolated" even with high Ct values. As a justification, the RKI points out that Ct values ​​can vary depending on the sampling and test procedure.

Christian Drosten: The virologist considers the Ct value to be meaningful under certain conditions. (Source: xim.gs/Getty Images)

Drosten recommends evaluating the Ct values

Charité virologist Christian Drosten had already recommended in September in the NDR podcast "Coronavirus Update" to use Ct values ​​to assess infectivity. If a standardized procedure is used, it is possible for every laboratory. Together with the test result, the findings can then be transmitted as to whether there is a high or low risk of infection from the person being tested.

If the health authorities take Ct values ​​into account in the future, this would save many non-contagious people from isolation and their families and friends from quarantine, said Drosten. Ultimately, the burdens would also be relieved for the offices, since in the case of non-contagious persons there is no need to follow up on the contacts.

Important NOTE: The information is in no way a substitute for professional advice or treatment by trained and recognized doctors. The contents of t-online cannot and must not be used to independently make diagnoses or start treatments.

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  • Coronavirus,
  • Viral infection,
  • Corona rapid test,
  • Corona pandemic,
  • SARS-CoV-2,
  • Covid-19,
  • PCR test,
  • Robert Koch Institute,
  • Christian Drosten,
  • Quarantine,
  • Virus,
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