Illustration of cycle threshold concept for quantitative PCR (qPCR)
Illustration of cycle threshold concept for quantitative PCR (qPCR)
With successive cycles of PCR (cycle number on the X axis), the fluorescence signal from fluorescently tagged nucleotides incorporated into the PCR products increases (gray line). The cycle threshold (Ct, dotted black line) is set at 10–1, where the signal is 10-fold greater than the background noise level of fluorescence seen with a negative control sample (blue line). In this example, the Ct of the test sample is 20 (dashed red arrow), meaning that 20 PCR cycles are required to reach the threshold fluorescence. A test sample with a greater amount of the starting material would reach the fluorescence threshold in fewer cycles and would have a lower Ct number.