The threshold is the numerical value assigned for each run that reflects the average dRn (change in fluorescence) during the initial cycles of PCR (baseline). The threshold is set by determining a statistically significant point (or value) above the baseline. You can manually adjust the threshold should you desire, to the place in the geometric phase where your replicates are tightest. For more information on how to manually set a threshold, please refer to the tutorial entitled Data Analysis on the ABI PRISM 7700: Setting Baselines and Thresholds (P/N 4370923). While this tutorial is based on the ABI PRISM 7700 system, the concepts are still valid for the Applied Biosystems real-time PCR instruments. You can search the document on our website by using the part number above as the keyword.
If you choose not to manually adjust the baselines and thresholds, the SDS software on the Applied Biosystems real-time PCR instruments has an Auto Ct algorithm that can be used to automatically generate baseline and threshold values for individual detectors. The algorithm calculates baseline and threshold parameters for a detector based on the assumption that the data exhibits the "typical" amplification curve. Experimental error (i.e. contamination, pipetting inaccuracies) can produce amplification curves that deviate significantly from a typical amplification curve. The data from these irregularities can affect the Auto Ct algorithm by causing it to generate incorrect baseline and threshold parameters for the associated detector. Therefore, Applied Biosystems recommends that after analysis of experimental data, you review all baseline and threshold parameters determined by the Auto Ct algorithm.
Find additional tips, troubleshooting help, and resources within our Real-Time PCR and Digital PCR Instruments Support Center.