What do the terms dual target PCR and Ct value mean?

Monday, 20 July 2020 | Uta Küsters, Dr. Peter Gohl

Reliable diagnostics with dual target PCR

In the dual target strategy, at least two independent gene regions are detected simultaneously in one examination: E-gene, N-gene, Orf1-gene, S-gene. This increases the sensitivity and specificity of the SARS-CoV-2 detection.

A positive result for at least 2 genes indicates a SARS-CoV-2 infection with a very high degree of certainty.

However, many courses of SARS-CoV-2 infected people do not always show such clear findings in relation to the PCR parameters: For example, the detectable virus quantity or nucleic acid decreases with healing or clearance. Often only one of the genes is detected. In some cases, control examinations over several weeks alternately showed a positive and then a negative result for one gene or for different genes.

Some recent studies have found that in these cases there is no longer a virus capable of replicating, but this cannot be completely ruled out at this point of time.

In our experience, we therefore also assess the isolated detection of a single gene as positive for SARS-CoV-2, depending on its specificity, but recommend a control in unclear cases.

 

Frequently discussed: the Ct value

Ct stands for threshold cycle and is a theoretical parameter in PCR analysis. The Ct value shows the point in time at which a reliable measurement signal is present in the course of the cyclic amplification: the lower the Ct value, the higher the amount of the target gene (corresponding nucleic acid) and thus the amount of virus particles. Conversely, a high Ct value indicates a low amount of nucleic acid and possibly a low virus titer.

The Ct value says nothing about the infectivity. Studies correlated a high Ct value (> 30) with a low growth rate of the virus in culture. In addition, the Ct value is also significantly influenced by the preanalytics such as smear location, quality of the smear and transport time.

We recommend taking further measures regardless of the Ct value.

If you have any questions about our corona diagnostics, please contact us: +49 6132 781-0