PCR and CT

PCR stands for Polymerase Chain Reaction. Polymerase is an enzyme by which any tiny genetic material (e.B. RNA fragments of viruses) is duplicated in order to detect the possible presence of these fragments. The chain reaction refers to the constant repetition of this multiplication. The multiplication causes an exponential doubling of the value. This means that the more cycles are performed, the test will be positive even when the virus load is originally the least clinically relevant. In other words, the higher the CT value (cycle threshold), the more likely a fragment can be found. The smaller the CT value (i.e. the threshold above which particles are detectable), the more likely a virus is to be. From a CT of 30, the cultivability of the virus decreases significantly. From a CT of 35, virtually no cultivable virus is detectable. [1]

PCR tests are therefore used to search for components of the genetic material of a virus and to demonstrate a possible presence. However, the test can be positive even if there are only virus fragments or inactive viruses. Different PCR tests search between one and three gene sequences. If only one gene sequence is used, the result is less accurate and the risk of a false positive test increases. The so-called Drosten test initially recommended by the WHO searched for one gene sequence. The tests used in Luxembourg measure up to three gene sequences.

The CT value is the number of repetitions (cycles) of the above-mentioned reproduction. In practical terms, it can be insinuated from this value how many genetic fragments were originally in the sample. The infectivity and severity of the disease depend, among other things, on the number of viruses present in the nose, mouth, throat, but also on the individual immune system of the affected person.

Infectious virus numbers strike at a CT value of 15-25. Here, someone is basically contagious. At 25 - 30 the risk of infection is lower, except with very close contact. With a CT between 30 - 35, there is a grey area in which it is statistically possible to be contagious, but this becomes increasingly unlikely. From a CT value of 35, infectious viruses are no longer present. However, due to the extreme accuracy of the test due to the reproductions, virus fragments can still be detected, even though they no longer have clinical relevance. The test is nevertheless positive.


[1] https://doi.org/10.1002/3527600418.bipcrbasicd0016