What is quantitative real-time PCR (qPCR)? Ct value and PCR efficiency

Real-time PCR is the ability to monitor the progress of the PCR as it occurs. Data is collected throughout the PCR process to quantitate DNA and RNA. By quantitation assays, it measures the amount of nucleic acid target during each amplification cycle of the PCR. The cycle of amplification that the fluorescence crosses the threshold to positive. The Ct value correlates with viral load. PCR efficiency is calculated by taking the number of amplified target DNA molecules at the end of the PCR cycle divided by the number of DNA molecules present at the beginning of PCR.

Review question: How do the two quantitation assays work? What does the Ct value represent for? What’s the acceptable PCR efficiency range?