2073. Toxoplasma gondii, avidity of IgG antibodies

The IgG avidity test for Toxoplasma gondii allows one to assess the duration of infection: acute or chronic. It is used to confirm the diagnosis, especially in pregnant women with positive or questionable IgM levels.

Determining the avidity of IgG antibodies to Toxoplasma gondii is a method for assessing the maturity of the body's immune response to toxoplasmosis. Avidity reflects the binding strength of IgG antibodies to specific antigens of the pathogen. In the early stages of infection, the body synthesizes IgG with low avidity, which gradually increases over several months.

Avidity testing allows us to differentiate between:

• An acute (recent) infection – low IgG avidity.
• A past (old) infection – high IgG avidity, indicating infection more than 3-4 months old.

This is critical during pregnancy: primary infection during gestation can lead to congenital toxoplasmosis, whereas latent infection before pregnancy usually poses no risk to the fetus.

The method is especially useful:

• In cases of positive IgM, when it is unclear whether the infection is recent or residual IgM from a previous infection.
• In cases of discordant IgG/IgM results in serological screening.
• When planning pregnancy if the woman is seropositive for T. gondii.
• In immunosuppressed patients, in whom reactivation of latent infection may be clinically significant.

Avidity is one of the most important criteria for differentiating the stage of infection and is part of the modern algorithm for managing patients with suspected toxoplasmosis. When combined with IgG and IgM tests, clinical assessment, and (if necessary) molecular diagnostics (PCR), it ensures accurate and safe patient management, particularly during pregnancy.

• Pregnant women – for screening for toxoplasmosis and assessing the risk to the fetus.
• Women planning pregnancy – to determine immune status.
• Newborns – if congenital toxoplasmosis is suspected.
• Patients with HIV, immunosuppression, or cancer – to assess the risk of reactivation.
• In cases of lymphadenopathy, fever, or unexplained eye or CNS damage.

• Venous blood

In order to exclude factors that may affect the test results, we recommend to follow the preparation rules: