What is the value of negative 1

What is the CT value and how is it obtained?

CT value is a term that one stumbles over again and again in connection with corona tests. In essence, it indicates how high the viral load is in a specific sample. Experts discuss what this could mean for a patient's infectiousness and the course of the disease. It's not easy: To really understand what the value says, it makes sense to take a closer look at the PCR test procedure.

The basis: How does a PCR work?

The PCR test is considered to be the most reliable way to detect an infection with the coronavirus. PCR stands for Polymerase Chain Reaction. It is a molecular biological process that is used to replicate DNA in order to analyze it later.

The purified genetic material is put into a plastic tube with various enzymes and DNA components, this is called the PCR approach. The tube goes into a thermal cycler - basically a combination of stove and refrigerator.

The thermal cycler can increase and decrease the temperature of the tube according to a programmed pattern. This is important because the replication of the DNA consists of different steps, each of which takes place at different temperatures and repeats in cycles.

The DNA double strand is repeatedly separated and supplemented so that the DNA doubles in each cycle. That is more or less all that happens in a normal PCR.

What are the special features of Corona-PCR?

The corona virus does not have DNA, but RNA. But that's not a problem, because there are enzymes that can convert RNA into DNA. The actual PCR is rewritten in one step before the start of the actual PCR. PCRs in which RNA is transcribed into DNA are called RT-PCR. The RT stands for the enzyme reverse transcriptase, which translates the RNA into DNA.

The corona test has another special feature. Normal PCRs simply duplicate the DNA, and the genetic material is then analyzed after the PCR. Corona-PCR contains fluorescence probes in addition to the normal approach. These probes are aimed at a specific target gene in the virus. Only when the probe has attached itself to the target gene does it begin to glow.

This glow can be measured with a photometer in the thermal cycler and read out on the computer. In this way, you can see during the PCR whether there is virus genetic material in the sample. Quasi in real time, which is why this form of PCR is called Real Time PCR.

How accurate are the tests?

The accuracy of the PCR tests in Germany is very high. Infections are detected 98 percent of the time. To be doubly sure, most samples are also examined for two different target genes with two different fluorescent probes.

Real-time PCR (this is abbreviated with a q or r - in the corona test, for example, RT-qPCR, sometimes also qRT-PCR) is a standard tool in virus diagnostics. It is also used, for example, to test blood from donated blood for viruses such as HIV or hepatitis.

The CT value can, but does not have to, say something about the viral load in the throat

Now what is the CT value?

CT stands for Cycle Threshold. It describes the number of cycles that a Real Time PCR has run before fluorescence can be measured. This takes longer, the less virus genome was in the plastic tube at the beginning of the PCR. A high CT value therefore indicates a low viral load in the sample.

What influences the CT value?

Unfortunately, a high CT value does not directly imply a low risk of infection for others. This is due to various factors:

  • lack of standardization

There is no uniform, standardized CT value above which a person who tested positive is no longer considered to be infectious. One of the reasons for this is that there are many different tests that are aimed at different target genes and whose PCR approaches differ slightly. This also differentiates which amount of virus leads to which CT value.

Studies by the Robert Koch Institute have shown that coronaviruses from a CT value of 30 can no longer be reproduced in the laboratory. This also speaks against the risk of infection from the person from whom the sample originates. Studies in Great Britain have shown, however, that even with a CT value of over 35, eight percent of the samples in the laboratory can still be reproduced. So these people could still be contagious.

Even the same test can produce different results in different laboratories. So there is no clear CT value from which a person is considered positive but not contagious.

Especially at the beginning of a corona illness, a person's viral load can still be very low. Anyone who does a test at the beginning of the infection that has a high CT value can be contagious just a few days later.

The result of a PCR test should therefore always be viewed in conjunction with possible symptoms. Risk contacts or staying in risk areas also play a role in the interpretation of the test.

  • The viral load in the sample is not necessarily the patient's viral load

A high CT value means that the sample examined contained few viruses. However, this does not mean that the patient himself only has a low viral load. Many factors can affect the amount of virus in the sample.

For example, how the smear was taken, how much sample material was taken, how the sample was stored and transported. In addition, there are fluctuations depending on the location of the smear.

Conclusion: The CT value does not say anything directly about the infectiousness of those affected. However, it is an important indicator when a doctor interprets the findings.

  • Hands off! - Where are the coronaviruses lurking?

    Contaminated doorknobs

    For a long time, researchers assumed that coronaviruses can be infectious on surfaces such as doorknobs for an average of four to five days. However, recent research shows that they can be infectious for up to 28 days - it mainly depends on the temperature. Viruses last the longest at 20 degrees Celsius. At 40 degrees only about a day.

  • Hands off! - Where are the coronaviruses lurking?

    Not that tasty!

    You should therefore be careful with lunch in the canteen - unless it has long been closed. In principle, coronaviruses can get on cutlery or dishes through an infected person sneezing or coughing directly. However, the Federal Office for Risk Assessment (BfR) writes that "no infections with SARS-CoV-2 via this transmission path are known to date".

  • Hands off! - Where are the coronaviruses lurking?

    Afraid of imported goods?

    Do parents have to fear possible infection from imported toys? According to the BfR, there have so far been no cases in which an infection from imported toys or other goods has been proven. Initial laboratory tests show that the pathogens can remain infectious for up to 24 hours on cardboard and up to three days on plastic and stainless steel.

  • Hands off! - Where are the coronaviruses lurking?

    Packets full of viruses?

    In general, human coronaviruses are not particularly stable on dry surfaces. Because the stability of the virus outside the human organism depends on many environmental factors such as temperature and air humidity, the BfR considers an infection by post to be "rather unlikely". However, the institute admits that more precise data on SARS-CoV-2 are not yet available.

  • Hands off! - Where are the coronaviruses lurking?

    Virus ping pong with your pet?

    Can my dog ​​infect me or I infect my dog? The risk of pets being infected with the coronavirus is assessed by experts as very low. But you can't rule it out. The animals themselves show no symptoms, so they do not get sick. However, if they are infected, it is possible that they transmit coronaviruses through the air they breathe or through excretions.

  • Hands off! - Where are the coronaviruses lurking?

    Dangerous vegetables?

    The BfR classifies the transmission of SARS-CoV-2 via contaminated food as unlikely. There are no proven cases of infection so far. Thorough hand washing before preparing a meal should be a matter of course - also in times before and after Corona. Because the viruses are sensitive to heat, heating food can further reduce the risk of infection.

  • Hands off! - Where are the coronaviruses lurking?

    Contaminated frozen food

    The previously known SARS and MERS coronaviruses do not like heat, but are quite insensitive to cold. They can remain infectious in the frozen state for up to two years at minus 20 degrees Celsius. However, the BfR gives the all-clear: So far, there has been no evidence of SARS-CoV-2 infection chains via the consumption of food, including frozen food.

  • Hands off! - Where are the coronaviruses lurking?

    Eating wild animals is prohibited!

    The COVID-19 outbreak has one good thing: China has banned the consumption of wild animals. There are many indications that the new coronavirus was transmitted from a bat to humans. The bat is not to blame for this; it had contact with humans in a Chinese market - probably against its will.

    Author: Julia Vergin