Cycle threshold values (Ct values) are the number of cycles or rounds of amplification needed to detect SARS-CoV-2.
When there is less virus in the sample, more rounds of amplification are needed which results in a
high Ct value. When there is more virus in the sample, fewer rounds of amplification are needed which results in a
low Ct value.
Ct values don’t tell us much in themselves without proper context. Ct values depend on many factors, examples include:
- Where the sample is collected from. For example, a nasal swab or saliva using the mouth rinse and gargle method
- When you get tested after exposure to COVID-19 (see image below)
- Sample quality
- Sample storage
- Sample transportation time.
The blue line in this image shows how the amount of SARS-CoV-2 viral material in your body can change over time. When there’s a low amount of virus in your body, the Ct value is high. This means more amplifications are needed to detect the virus.