Corona studies help to better classify the Ct value
Anyone who tests positive for the SARS-CoV-2 coronavirus will wonder how long they are contagious. If a PCR test is also carried out, we also receive the so-called Ct value. This is then referred to as a "strong" or "weakly positive" test. Do studies support these statements?
What is the Ct value?
The result of a PCR test is usually not only "positive" or "negative", but also includes the so-called Ct value - where "Ct" is the abbreviation for "cycle threshold". The term is derived from the way a PCR test works. If an acute corona infection is suspected, a PCR test can be used to reliably detect the smallest amounts of the virus's genetic material in the laboratory.
The PCR (polymerase chain reaction) is the gold standard among corona tests and at the same time the most complex procedure. The sample is taken from the nasopharynx of the affected person. This means that even the smallest amounts of the virus's genetic material can be multiplied in several cycles in the laboratory.
What does the Ct value say?
The Ct value is a measure of how much viral genetic material (in the case of coronavirus, “RNA”) is present in a sample, i.e. it indicates the measured viral RNA concentration. If this is low, the laboratory needs several amplification cycles to be able to detect it in the PCR. The more PCR cycles that have to be completed for detection, the lower the coronavirus RNA concentration of the sample being examined. What is being sought is the exact threshold at which the genetic material can be detected for the first time. So, if the Ct value is low, the viral load is high. If it is high, the viral load is low.
Overview of the significance of Ct values
Ct value below 20, 30 or above
Ct values above 30 were considered in the Corona pandemic to indicate a low viral load that probably no longer leads to infection. On this basis, patients in hospitals were sometimes taken out of isolation at this value and no longer shielded from other people. However, after looking at the current study situation, this interpretation of the Ct value should be treated with great caution.
Studies: Relevance of the Ct value
The PCR test is the most accurate of the available corona tests. However, this method also has limitations; false-negative test results are possible. False-negative describes the case where the person being tested is negative according to the PCR result, but in reality has a corona infection.
At the moment of testing, it is often not clear at what point in the course of the disease the person is. The Ct value can then be high (and the infectivity therefore supposedly low), but the person may only be at the beginning of the disease. False-negative PCR results are also possible in the late phase of the corona infection. Throat swabs can be virus-free if the infection has migrated to the lower respiratory tract and there is an infectious viral load below. A high value can therefore mean that the infection is just beginning or is already well advanced.
The quality of the swab can vary and differences can arise depending on the tube or swab used. In addition to this inadequate sample quality, there are different PCR test kits and laboratory practices, and transport or storage can be faulty. This can also lead to varying Ct values. Therefore, the Ct value should be seen as a trend and not used in isolation to assess the risk of infection.
According to the Robert Koch Institute, the interpretation of borderline or unclear Ct values requires careful evaluation by experienced specialists in order to make clinical decisions based on this Ct value interpretation. It must therefore also be used with caution in clinical practice.
Ct value in vaccinated people
With a full corona vaccination and a later booster vaccination, people can significantly reduce their risk of becoming seriously ill with COVID-19. The risk of developing symptoms is also lower. Nevertheless, so-called vaccine breakthroughs - i.e. when a fully vaccinated person is diagnosed with SARS-CoV-2 infection with symptoms - are not uncommon, especially with Omikron.
The symptoms of a breakthrough infection are similar to the COVID-19 symptoms of an unvaccinated person, but are milder. According to studies, severe courses are much rarer. Vaccination is therefore still worthwhile.
Study results on the mean Ct values in unvaccinated and vaccinated infected people are currently only available for earlier virus mutations (such as the delta variant). Accordingly, vaccinated infected people can also reach Ct values that are associated with a relevant risk of infection for other people. Nevertheless, the vaccination appears to have a positive effect on the viral load and thus the Ct values. However, further studies are still needed to determine whether this positive "vaccination trend" in relation to the Ct value will continue with the Omicron mutation. The Ct value alone should not be used to decide whether someone is still contagious to other people.
Duration of infection after COVID-19 infection
Transmission of the virus to others is unlikely after 7 to 10 days of illness. This applies to people with a functioning immune system. Exceptions prove the rule, so caution is advised - even after a booster vaccination.
What the Ct value says about infection
It is certainly true that most people with a high Ct value in their PCR are potentially less contagious to those around them. Nevertheless, the Ct value is less important than was long assumed. Because no one can say with certainty what phase of the disease the affected person is in, what quality the sample collection, transport and storage were carried out with, and what test settings the respective laboratory used. A pleasingly high Ct value can actually be significantly lower.
Test out of quarantine?
The best way to be sure not to infect anyone remains to test yourself out of isolation or quarantine with a negative test, but there is no longer a mandatory quarantine.
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The contents of this article reflect the current scientific status at the time of publication and were written to the best of our knowledge. Nevertheless, the article does not replace medical advice and diagnosis. If you have any questions, consult your general practitioner.
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