Displacement of one ligand by another calculated from the reverse displacement of the second ligand by the first

This paper relates a generally applicable method for calculating mean displacement of a ligand, B, by another ligand, D, when the reverse displacement of D by B is known. The method is applied to calculating displacement of bilirubin from binding to albumin by a drug, warfarin or ceftriaxone. A binding isotherm for the drug to albumin is drawn, as the logarithm of free drug vs total drug concentration. A second binding isotherm is drawn for the drug in the presence of a certain concentration of bilirubin. The area between the two curves, up to a given total drug concentration, is measured and is divided by the concentration of bilirubin. The antilogarithm of the ratio is equal to the geometric mean of the ratio of free concentrations of bilirubin in the presence and absence of the drug when the bilirubin concentration varies from zero to the given value. The calculations are verified by comparing the direct determinations of the bilirubin displacing effect of the drugs by peroxidase kinetics.