THE AVIDITY TEST The power to discriminate between past and recent infection in Toxoplasmosis and CMV patients

Avidity is defined as the strength with which the IgG attaches to antigen, IgG avidity matures with the length of time following primary infection. Thus, IgG produced within the first few months following primary infection exhibits low avidity, whereas IgG produced several months or years later exhibits high avidity.

The measurement of IgG avidity has shown its power in various clinical settings, especially in situations where timing and differentiation of primary and secondary infections is crucial. This is especially true for CMV and Toxoplasmosis.

The debilitating effects of congenital CMV infection are strongly linked to maternal primary infection, rather than reinfection or reactivation, during pregnancy. IgG antibodies persist lifelong and IgM can be detected for > 1 year after infection. Thus, the challenge to the physician caring for a pregnant woman with clinical or serologic evidence of CMV infection is to determine if the infection represents a newly acquired (primary) infection, or represents either reinfection or reactivation of a pre-existing (nonprimary) infection. Determining the time of primary infection can help guide decisions regarding antiviral therapy by identifying those women who should or should not be treated during pregnancy.

Avidity testing also applies to toxoplasmosis testing in pregnancy where establishing timing of infection is critical for the management of the pregnancy. IgG antibodies persist lifelong and IgM can be detected for< 1 year after infection. Demonstrating that the IgG antibodies are of high avidity can establish that infection was acquired before conception and there should be no risk to the foetus unless there is immunocompromise.

The IgG avidity test is a confirmatory test and should not be used alone. When used appropriately and interpreted with other serological tests, it is a very useful test.

The Avidity test is now available at alfa Laboratories