Important for the free test: What does the Ct value in a PCR test mean?

Friday 14 January 2022

Important for free test
What does the Ct value in a PCR test mean?

By Klaus Wedekind

A positive PCR test does not always mean that you are contagious, and a Ct value level is crucial. ntv.de explains what exactly it is, what is important and why value can play an important role in a free test.

PCR tests are the best tool when it comes to detecting COVID-19 infection. But when the incidences are as high as they are now in the omicron wave, it is not always the best option. On the one hand, resources are scarce, on the other hand, tests are very sensitive when it comes simply to determining whether a person is contagious or not. A positive result does not always mean that you are contagious. The viral load, which is determined by the so-called Ct value, is crucial for this.

What does the value of Ct indicate?

in the polymerase chain reaction (soligomerase clittle sreaction) usually in the laboratory by taking a swab from the nose or throat. Ct value (cycle-TThe hrshold value indicates the number of diffusion cycles necessary for the Sars-CoV-2 genome to be measured. It follows that a higher Ct value means a lower viral load in the sample and vice versa.

Viral load, in turn, is critical when it comes to determining whether or not a person is contagious. To determine this, the amount sufficient in the laboratory is used for the growth of the pathogen. The RKI assumes that there is an infection of 30 cycles, i.e. a Ct value less than or equal to 30. This is evidenced, among other things, from the isolation release criteria, which the authority allocates to clinicians guidance be.

High value does not necessarily mean safety

However, a Ct value greater than 30 does not automatically mean that the person tested is not contagious. Among other things, it depends on whether the smear was performed correctly. And even if this were the case, each removal would result in different amounts of genetic material and therefore different Ct values. The nose and throat may also not be the right place to take the sample. according to LADR Labs Network Dr. Kramer and colleagues In the case of pneumonia (inflammation of the lungs), the virus reproduces mainly in the lower respiratory tract.

In addition, the sensitivity and detection limit of different PCR methods are different and will not yield exactly the same results with the same smear. Therefore, under the leadership of the RKI, two samples with standard amounts of virus were sent to different laboratories for comparison and the results were then evaluated. According to LADR, 22 to 27 cycles were required for detection. RKI took obfuscation into account, which is why a Ct value of 30 includes a security buffer.

It depends on the timing

Most problematic is that the viral load is also low soon after infection and the test can lead to an elevated Ct value and a negative result. But because SARS-CoV-2 replicates very quickly in the body, a person tested can be highly infectious even after a few hours. In the case of asymptomatic people, a second PCR test is always necessary with a sufficient period of time. This is time consuming and is one reason why less accurate, but faster and cheaper antigen tests are used, especially when resources are scarce to perform random checks.

Conversely, a Ct value much less than 30 does not necessarily mean that you are contagious. The test detects only the genetic material of Sars-CoV-2, from which a corresponding amount of the virus is extracted. Despite the positive result, the immune system can eliminate the same pathogens for a long time.

Important value for free test

The value of Ct also plays a role in the determined probability of a Quarantine or isolation It is shortened by PCR test. If this is negative, then everything is clear. But even with a positive result, the health authorities have the option to allow the release of “isolation” if the Ct value is less than 30. However, before seven days have passed, you cannot test yourself for free to ensure that you are not in the early stages of infection.

However, one cannot necessarily rely on the determination of the viral load that is taken into account. “Assessing the risks of infection and therefore issuing and removing the quarantine order is the responsibility of the responsible health authority in each individual case,” RKI writes. Of course, it is also a prerequisite that the value of Ct be reported in the test result, which is obviously not (yet) always the case.