Ans. is B. IgG is chronic IgM is acute Ref- Distinguishing of acute and chronic phases of toxoplasmosis has critical importance in pregnant women and immunocompromised patients. Some assays have been used to measure Toxoplasma specific IgM antibodies as indicators of the acute phase infection . Toxoplasma IgM antibodies can be detected for 1 year or longer in some cases, so in asymptomatic individuals with stable titers of Toxoplasma IgG antibodies, positive IgM results are not easy to interpret . The IgG avidity assay that had been started in the 1980s for diagnosis of rubella and hepatitis C is now used to distinguish between acute and chronic toxoplasmosis . This method was originally developed by Hedman et al. in Finland. It is based on dissociation of hydrogen bond between antigen and antibody with urea . High avidity (AI>=60%) means that Toxoplasma infection was acquired before 3 months ago, whereas borderline avidity (50%=60%). It was suggested that Toxoplasma specific IgG avidity assay that is performed on specific IgM positive samples could diagnose acute and chronic phases of toxoplasmosis . Candolfi et al. also demonstrated that measuring of IgG avidity could differentiate between acute and chronic phases of toxoplasmosis. Our results showed that there was no significant relation between the concentration of IgG antibody and the level of AI. Therefore, the strength of links between antibody and antigen is not dependent on antibody concentration, which was confirmed by other investigators . Meanwhile, there are a converse relationship between IgM titer and levels of IgG AI in this study (P<0.05). On the other hand, Remington et al. suggested that IgG avidity ELISA test is not enough to confirm acute toxoplasmosis, and it should be done in addition to IgM ELISA test. Many investigators had mentioned the role of IgM detection as a key for diagnosis of acute toxoplasmosis . One of the limitations of IgG avidity method is detection of reactivated toxoplasmosis in immunocompromised patients . In the present study, both IgM ELISA and IgG avidity ELISA tests together could confirm the phases of infection absolutely.