A RT-PCR test came positive and the CT value is 27, which means apparently a low contagion level. But is there a way to estimate when (in average) it could go til 35 (the cutoff)?

The graph from here show the correlation between CT and number of days of infection.

enter image description here

Also, some of my findings show that at CT value about 26, there is a plateau phase reached:

enter image description here

Can we conclude, from CT value reading of 27, that the PCR test would come negative as soon (the cutoff 35 for the test method)?

In my understanding it was because if it already at 27 cycles of reproduction, in like 4 days it will reach 35.

P.S. It goes with no explanation that my medical understanding is quit limited in this domain

1 Answer 1


It is difficult to be sure from your question, but you may have a slight misunderstanding of "Ct" value.

Ct stands for cycle threshold¹. In a real time polymerase chain assay, a sample is analyzed using fluorescent probes which generate light as more copies of viral genetic material are created.

This image adapted from Raaban et al (under CC-BY 4.0) shows how this works: enter image description here

Thus, as fewer cycles are required to detect the signal, more viral genetic material was present in the initial sample.

Some authors have described Ct values of 17-24 as "high viral load" and 25-35 as "moderate viral load"², but each test manufacturer is likely to set their own cutoffs for their specific assay.

While these is relatively good evidence that lower Ct value correlates with more severe SARS-Cov-2 disease¹, there is not a direct relation between Ct value and viral load in the patient. Other factors might include how the sample was collected and diluted. As Raaban and colleagues note:

The pre-analytic variables include collection technique, specimen type, time of sample taken and viral kinetics, the difference between the viral load in URT and LRT samples, transport and storage conditions before the testing, and the specimen age.

Thus, there is no reliable way to determine how Ct value will change over time. It might increase if the virus is being successfully fought by the immune system. However, the Ct value might decrease if the virus continues to successfully replicate.

  • @Vega While you're editing, it would be helpful to add some evidence of prior research. Questions here are required to demonstrate prior research.
    – Carey Gregory
    Commented Dec 30, 2021 at 2:15

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