In qPCR, what is a Ct value, and what does it indicate?

• A. The cycle number at which fluorescence crosses a defined threshold; higher Ct means more starting template.
• B. The fluorescence intensity at the threshold.
• C. The cycle number at which fluorescence crosses a defined threshold; lower Ct means more starting template.
• D. The total fluorescence accumulated by the end of the run.

Answer: (C)

In qPCR the main idea being tested is how the Ct value relates to how much starting template you have. Ct stands for the cycle number at which the fluorescence signal crosses a predefined threshold during the exponential phase of amplification. This threshold is set so that the signal reflects true exponential growth of the product, making the Ct value a reliable indicator of how much template was present to begin with.

Why the chosen statement is best is because a lower Ct means you reached the threshold earlier, which happens when there is more starting template. If you start with a lot of target molecules, only a few cycles are needed to accumulate enough product to cross the threshold. If you start with little template, you need many more cycles, so the Ct is higher. This inverse relationship is fundamental to how qPCR quantifies DNA.

It's also important to distinguish Ct from other measurements. The fluorescence intensity at the threshold is simply the signal level used to define crossing the threshold, not the Ct itself. The total fluorescence accumulated by the end of the run doesn’t directly indicate how much template you started with; Ct is about when the threshold is reached, not how much signal accumulates overall.

In practice, Ct values are compared either relative to a standard curve to estimate absolute amounts or between samples using methods like ΔCt or ΔΔCt to assess relative differences in starting template.