Parallel-logistic potency assays

Parallel-logistic potency assays use the entire sigmoidal curve to represent dose-response correlations and calculate relative potencies.

This assay type is most commonly used with the 4-parameter logistic fit function. Its four parameters describe a symmetric sigmoidal curve with the upper asymptote given by the A parameter and the lower asymptote by the D parameter.

Variants are the 5-parameter fit function, which adds an asymmetry parameter (G), and the 3-parameter fit function, which is structurally identical to the 4-parameter logistic fit but keeps either the lower or upper asymptote at a fixed value or at the mean of a control line.