However, this is not true if different instruments, reagents, primers and probes, or reaction volumes are involved in producing the two C ts. the other, could be valuable in concluding that there is less template in the first sample, assuming all other factors such as instruments, reagents, and assays are equal. Generally, an efficiency between 90 and 110% is considered acceptable. The PCR efficiency is dependent on the assay, the master mix performance, and sample quality. In this example, although the high-efficiency condition (the blue curve in Figure 5) gives a later C t at high concentrations, it results in better sensitivity at low target concentrations. In Figure 5, two samples (X and Y) amplified under low and high efficiency conditions show different C t values for the same target concentration.
A dilution series amplified under low efficiency conditions could yield a standard curve with a different slope from one amplified under high efficiency conditions. The efficiency of a PCR reaction can also affect C t. Therefore, the C t values from PCR reactions run under different conditions or with different reagents cannot be compared directly. However, artifacts from the reaction mix or instrument that change the fluorescence measurements associated with the C t calculation will result in template-independent changes to the C t value. The C t value increases with a decreasing amount of template.
The threshold must be set in the linear phase of the amplification plot in Figure 1C. The exponential phase in Figure 1B corresponds to the linear phase in Figure 1C.
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We will discuss the most common template-independent factors that can influence C t and describe how to evaluate the performance of a real-time PCR reaction.įigure 1, above, shows several parameters of the real-time reaction amplification plot.
Many factors impact the absolute value of C t besides the concentration of the target. It is a relative measure of the concentration of target in the PCR reaction. C t (threshold cycle) is the intersection between an amplification curve and a threshold line ( Figure 1B).