Figure 1.

Definition of C_q and ΔC_q. When amplification curves are plotted on a logarithmic fluorescence axis, the exponential phase, with a constant PCR efficiency, can be identified as the cycles where the fluorescence values are on a straight line (solid lines; Eff. = 1.91). The intercept of the downward extrapolated line with the Y-axis is the fluorescence associated with the starting concentration (N₀). Setting a quantification threshold (N_q, horizontal green line) is used to determine the C_q value, defined as the number of amplification cycles needed to reach the threshold (vertical green arrows). The uniform blue and orange triangles, from N₀ to N_q to C_q, illustrate that for two sets of reactions with about 10-fold different starting concentrations and the same PCR efficiency; the difference in C_q values is proportional to the difference in log(N₀). Therefore, the difference in C_q values (ΔC_q = C_q,1 - C_q,2) is proportional to the log of the starting concentration ratio (log(N_0,2/N_0,1). The dotted lines illustrate that the same ratio of N₀ values and an assay with a lower PCR efficiency (Eff. = 1.47) lead to higher C_q values (C_q,1* and C_q,2*) and a larger ΔC_q*. The graph is based on actual amplification data; the dotted lines represent a hypothetical less efficient assay.