Neutralization assay to resolve discrepancies between positive results in new highly sensitive anti-Toxoplasma gondii IgG assays and negative results in reference tests.

High sensitivity of IgG assays is required in order to diagnose Toxoplasma infection in immunocompromised patients and women before and during early pregnancy. Positive results obtained in new highly sensitive Toxoplasma IgG assays have been observed in sera that gave negative results in reference assays. To allow the resolution of these discrepancies, we developed a neutralization assay using 80 microg/ml of a soluble membrane fraction of Toxoplasma in the automated Elecsys IgG assay. We were able to block the detection of Toxoplasma IgG antibodies in 283 sera with IgG titers ranging from below the limit of detection to >2,300 IU/ml. The mean percentage of neutralization was 88%, with a minimum neutralization of 77%. In summary, the neutralization assay represents a valuable tool to confirm the specificity of Toxoplasma IgG antibodies in the sera of immunocompromised patients and women before and during early pregnancy. Discrepancies between positive results in highly sensitive Toxoplasma IgG assays and negative results in reference assays can, thereby, be resolved and may allow a decision to be made on the management of patients.