Cycle Thresholds (CT)

This section is designed to guide you on Cycle Thresholds (CT), which are guidance on viral load. Click here for general information on the Coronavirus Molecular Test (PCR), instead information on cycle thresholds.

What is the CT or cycle threshold?

The cycle threshold or CT (Cycle threshold) is an indicator that allows us to correlate the analysis of your sample with the amount of viral genetic material detected or viral load. The molecular test processing uses cycles to detect the virus. The CT is defined as the number of thermal cycles in which the signal passes the positivity threshold, this indicates how many cycles were necessary to perform to detect the virus. The higher the amount of virus, the fewer cycles will be required to detect it and the patient will receive a lower CT, and vice versa.

Why would it be relevant to know the viral load?

Associations of high viral loads with more severe clinical pictures and low viral loads have been described in cases of asymptomatic patients or with mild symptoms. Viral load can be a determining factor in determining the level of contagiousness.

Is the CT value decisive for the evolution of a patient's clinical picture?

No, it is not determinative. CT and viral load do not determine how a patient will progress clinically. Each organism is different and the most important thing is that your treating doctor is aware of your result so that he or she can evaluate your case and monitor your clinical picture, in such a way that you can access medical attention if you require it in any situation.

Can the CT value determine the time of infection?

No, the CT value may be different in each case. If a person was infected with a high viral load, their CT will likely be lower than that of a patient who was infected with a low viral load, regardless of the time of infection. This determination does not allow the time of illness to be established. The time of illness is determined by evaluating the onset of the patient's symptoms and their intensity. In asymptomatic patients, the time of illness may not be able to be determined.

How does viral load change over time?

It can be observed that the viral load has a very rapid increase during the two to three days prior to the onset of symptoms, reaching its peak on the day of symptom onset. Later there is a gradual decrease in viral load 1.

Are CT values comparable between different laboratories?

No. It is difficult to make comparisons between the different molecular tests. For this reason, the use of CT has not yet been validated in a standardized way in clinical management guidelines for Covid 19 cases.
CT values can vary based on a number of factors, including sampling and the use of different equipment and reagents.

Bibliographies:

1. Understanding cycle threshold (Ct) in SARS-CoV-2 RT-PCR. A guide for health protection teams. Public health England. Available on: https://assets.publishing.service.gov.uk.
2. Ilka Engelmann et al. Preanalytical Issues and Cycle Threshold Values in SARS-CoV‑2 RealTime RT-PCR Testing: Should Test Results Include These? ACS Omega 2021, 6, 6528−6536. Available on: https://dx.doi.or/.