ELISA (also k-ELISA) can be used to measure the dissociation constant (Kd), which is commonly applied to describe affinity . According to the mathematical model presented above, the Kd value can be calculated via pre-incubation of a receptor with varied concentrations of ligand and subsequent detection of the soluble receptor (phage) via ELISA. This approach was adapted and applied to evaluate the affinity of several phage-derived ligands , but without a theoretical analysis and fulfilling fundamental Friguet's assumptions . In these papers, phages were treated as monovalent receptors while they are tri to pentavalent (due to a varied number of pIII protein per phage particle). Moreover, the authors applied a wide range of ligand concentrations, high excess compared to the receptor and treated the concentrationsignal function as logarithmic, while the discussion above concluded it is not possible to apply Frigeut's method for almost completely saturated receptors (due to ligand excess).