Relative potency (RP) is defined as ‘The ratio of two equally effective doses of the test preparation (T) relative to that of the standard (S).’ Finney 1964 Relative potency assays involve several dose groups of each of T and S: referred to within QuBAS as the ‘test sample’ and the ‘reference standard’. A readout, or ‘response’, is measured for each individual experimental unit (well or animal).
Figure 1. illustrates the log(dose):response relationships for the two samples. The horizontal distance between these is the difference between equally effective log doses, or, equivalently, the log of the ratio of two equally effective doses: log(RP).
See Figure 1. where this distance is denoted by Δ. The RP is calculated by back-transforming Δ.
The RP is only a valid measure of the potency of a test sample if its dose-response curve is parallel to that of the reference: that is, one is a horizontal shift of the other. This is because if the curves are not parallel, the horizontal distance varies depending on the response chosen for the comparison and therefore the RP is not uniquely defined.
Figure 1: Parallel dose-response curves
Confidence intervals for the relative potency and other functions of model parameters (for example slope ratios) are calculated in QuBAS using the Delta method or Fieller's theorem.
