2140. Rubella virus, avidity of IgG antibodies

An IgG avidity test for rubella virus allows one to determine the duration of infection – whether recent or previous. It is important in cases of positive IgM, especially in pregnant women, to assess the risk of congenital infection in the fetus.

Antibody avidity is the ability of IgG to tightly bind to an antigen. After a primary infection, the body initially synthesizes IgG with low avidity, but over time, antibodies 'mature', and their avidity increases. This is why determining IgG avidity to the rubella virus allows us to differentiate acute (recent) infection from past infection.

This is especially important in patients with positive or equivocal IgM, when it is unclear whether the infection is active or whether the IgM are residual from a previous illness or vaccination. In pregnant women, this is critical for assessing the risk of congenital rubella in the fetus.

Examples of clinical significance:

• Low IgG avidity indicates recent infection – usually within the first 2-3 months after infection.
• High IgG avidity indicates a recent infection or vaccination – the risk to the fetus is minimal.
• Intermediate avidity requires follow-up or repeat testing in 2-3 weeks.

The avidity test helps:

• Reduce the number of false-positive diagnoses of acute infection.
• Avoid unnecessary termination of pregnancy.
• Provide a more accurate interpretation of the TORCH complex.

Rubella IgG avidity is a key test in interpreting a positive IgM result, allowing physicians to make informed clinical decisions in sensitive situations such as pregnancy or conception planning. When combined with IgG, IgM, and clinical data, this test provides a complete picture of infection status.

• Pregnant women with positive IgM antibodies to rubella virus – to determine whether the infection is active.
• Women planning pregnancy with questionable serological status.
• Patients with positive IgG and IgM antibodies to determine the phase of the infection.
• In cases where acute infection must be ruled out after vaccination or exposure to an infected person.
• For retrospective assessment of the risk of fetal infection if the diagnosis is made later.

• Venous blood

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