Clinical Applications of Cytomegalovirus IgG Avidity Testing

Cytomegalovirus (CMV) IgG avidity, defined as the strength with which IgG binds to an antigen, matures gradually following primary infection. Low avidity indicates recent infection (within the prior 3 months), whereas high avidity indicates infection >6 months earlier. In adults, IgG avidity testing is better than CMV IgM testing for identifying primary infection, since IgM persists for many months in some individuals. The key clinical application of CMV IgG avidity testing is to identify primary CMV infection during pregnancy, which is associated with increased risk for intrauterine transmission. A related application is diagnosis of primary CMV infection in children less than 1 year old. In transplantation, avidity testing helps identify the cellular source of CMV IgG in seroconverting solid organ recipients; in seropositive bone marrow recipients, avidity results serve as an indicator of successful engraftment. An unrealized application is assessment of CMV IgM⁺ IgG⁺ organ or blood donors; many are IgM⁺ due to IgM persistence and may thus still qualify as donors. This article provides a review of CMV IgG avidity methodology, as well as clinical applications.