Ct (threshold cycle) is the intersection between an amplification curve and a threshold line. It is a relative measure of the concentration of target in the PCR reaction. Fold change for each target gene is calculated in the following way: DeltaCt(gene) = Ct(gene, sample) - Ct(gene, control)
What is a CT in qPCR?
In a real time PCR assay a positive reaction is detected by accumulation of a fluorescent signal. The Ct (cycle threshold) is defined as the number of cycles required for the fluorescent signal to cross the threshold (ie exceeds background level).
What is relative quantification?
In absolute quantification using the standard curve method, you quantitate unknowns based on a known quantity. In relative quantification, you analyze changes in gene expression in a given sample relative to another reference sample (such as an untreated control sample).