The image on the right illustrates a result from a real-time PCR reaction. The y-axis is the level of fluorescence and the x-axis is the PCR cycle number. In this reaction there is a control reaction with a baseline of 30 fluorescence units (shown as a solid blue line) and the patient sample. By looking at the graph you can see that the sample exceeds the baseline fluorescence point at 18.93 cycles. This is the cycle threshold (Ct). The lower the Ct value the higher the original concentration of DNA, which requires fewer PCR cycles to build enough amplicons to cross the pre-established threshold.
