Avidity

Avidity (from Latin avidus, "greedy") refers to the strength of the antigen-antibody binding after the formation of the antigen-antibody complex. Avidity is composed of the number of binding sites and their affinity (antibody valence). It is not relevant whether the binding is specific or multispecific. It is multi-specific if an antibody reacts with different antigenic determinants(cross-reactivity).

The determination of avidity is important for the clarification of diagnostically ambiguous cases, such as reactivations, persistent IgM-AK, polyclonal stimulation. (Prince HE et al. 2014)

Monovalent interactions have a low avidity, bivalent ones a high avidity and polyvalent ones a very high avidity. Low-avidity antibodies are present when the ELISA absorbance is significantly reduced by urea treatment. The relative avidity index (RAI) is calculated from the measured values with and without urea treatment.

1. Prince HE et al (2014) Role of cytomegalovirus (CMV) IgG avidity testing in diagnosing primary CMV infection during pregnancy. Clin Vaccine Immunol 21:1377-1384.
2. Suligoi B et al (2017) HIV avidity index performance using a modified fourth-generation immunoassay to detect recent HIV infections. Clin Chem Lab Med 55:2010-2019.