IgG Avidity Test
Definition
Measures how tightly IgG antibodies bind to their target. Binding strength is low in the early period of an infection and increases over time; for this reason it provides information about when an infection was contracted.
Purpose
Answers a question that is especially critical in pregnancy: was the infection contracted before pregnancy or during pregnancy?
Why it is needed: IgM antibodies against Toxoplasma alone often provide insufficient information; a complementary method is needed to reach a diagnosis from the first sample. Because IgM can remain positive for months, up to 18 months in Toxoplasma, it can be mistaken for a "new infection".
Results
| Avidity | What it suggests |
|---|---|
| High | Infection is older, contracted further in the past |
| Low | Infection may have been contracted recently |
In immunocompetent pregnant women, the IgG avidity test has been able to distinguish acute from chronic infection on the first serum sample; in an evaluation of 91 people, sensitivity was 100% and specificity was 92.7%, with both reaching 100% in immunocompetent women.
Risks and Limitations
In immunosuppressed people or during ongoing treatment, the result must be interpreted with caution, and other serological markers including IgA should also be checked in these situations.