When k3/k4, or an estimate of it (usually BPND or Bound/free ratio at pseudo-equilibrium), is measured under (at least) two different ligand concentrations, the data can be plotted against the concentration of free ligand (tracer ligand + possible drug). The high ligand concentration can be achieved either by injecting the labeled ligand with large amount of cold ligand (low specific activity), or by giving another inhibitor, for instance an oral dose of a drug compound that binds to the same receptor.