Ans. is 'd' i.e., Avidity testing must be done to differentiate b/w IgA & IgM Avidity test is used to measure the strength of antigen-antibody reaction for IgG antibody (not to differentiate betweenIgA & IgM) Toxoplasma immunoglobulins to diagnose congenital toxoplasmosis 1) IgG o Presence of IgG in infants's blood does not confirm the diagnosis of congenital toxoplasmosis because IgG can easily cross the placenta. So, one cannot predict whether the IgG is of maternal origin or due to fetal infection. o Transplacental IgG antibody disappears by 6-12 months of age. Serological tests to detect IgG : - i) Sabin-Fieldman dye test (Most preferred IgG test) ii) 1gG IFA test (IgG indirect fluorscent antibody test) iii) Differential agglutination (AC/HS) IgG test. iv) Avidity test : - It measure the strength of antigen-antibody reaction for IgG antibody. 2) IgM o IgM does not cross the placenta; therefore, IgM is the test of choice for determining congenital infection. o Serological tests for IgM are : - i) IgM IFA (indirect fluorscent antibody test) ii) Double sandwich enzyme linked immunosorbant assay : - More sensitive than IgM IFA. iii) immunosorbant agglutination assay (ISAGA) 3)IgA o IgA may have greater sensitivity for neonate compared to IgM assay. o Serological tests for IgA are : - i) Double sandwich enzyme linked immunosorbant assay (IgA ELISA) : - It is better than double sandwich IgM ELISA for diagnosis of congenital toxoplasmosis. The immunosorbant agglutination assay (ISAGA) IgA.