The Ct or threshold cycle value is the number of amplification cycles a PCR test needs to perform on a sample before the fluorescence generated crosses the fluorescence threshold and is significantly above the background fluorescence.

At the threshold cycle, a detectable amount of amplicon product has been generated during the early exponential phase of the reaction.

The Ct is inversely proportional to the amount of RNA in the sample – if there is a low amount to start with it takes more cycles to amplify it enough to generate a signal.

If the sample has a high level of RNA (high viral load) it takes fewer cycles to generate a signal.