A new approach to differentiate recent vs chronic Toxoplasma infection: avidity ELISA in Toxoplasma serology

The diagnosis of toxoplamosis during pregnancy is based on maternal serology, due to the asymptomatic nature of the disease. Detection of specific IgM although is the method used all over the world to detect acute infection, persistence of IgM for long periods poses problems in distinguishing acute from chronic infection, which is of crucial importance in pregnancy. Avidity ELISA is a method recently developed to distinguish IgG antibodies developed at an early stage of infection from those that reflect past immunity. The avidity assay uses protein-denaturing agents and is a modification of an ELISA. The usefulness of this technique was tested on sera of 113 pregnant women screened for Toxoplasma specific IgG/IgM antibodies. Nine of the sixteen sera positive for IgM/IgG antibodies and three sera positive for IgG alone were subjected to avidity ELISA. Only three sera were positive for low avidity IgG indicative of recent infection. All the three sera positive for IgG alone showed high avidity.