A qualitative or quantitative serological assay measuring the level of IgG antibodies directed against Cytomegalovirus (CMV) in blood serum. The immune system synthesizes these antibodies weeks after an initial CMV infection, and they typically remain detectable in the bloodstream for a lifetime, serving as an immunological footprint. Their presence confirms historical exposure to the virus but does not independently clarify whether the infection is currently active, a past resolved infection, or in a latent state.