Which value of antigenic valency should be used in antibody avidity calculations with multivalent antigens?

The calculation of antibody avidity for multivalent antigens requires the experimental determination of the antigenic valency by extrapolation of Scatchard plots of the type f/d versus f. At any ratio of the reactants, the antigenic valency corresponds to the maximum number of antibody molecules that can be bound simultaneously. In spite of the steric hindrance effect that is prevalent with multivalent antigens, the antigenic valency is a constant parameter and not a variable influenced by the ratio of antibody to antigen.