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IgG avidity test (Toxoplasma, CMV, rubella)

TORCH · also called Avidity index

What it measures

Measures how strongly IgG binds its antigen. Avidity is low after a recent primary infection and matures over months, helping date infection when IgM is positive.

Specimen and preparation

Specimen: Serum
Tube: Gold top

Reference range

Group	Range
High avidity	Above kit cut-off (often > 0.3 or > 60%)

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

High avidity: infection more than about 3–4 months ago (e.g. before pregnancy if tested early)
Borderline: cannot date infection, repeat or refer
Used with a positive IgM to guide fetal testing decisions

Causes of low values

Low avidity: recent primary infection (within about 3–4 months)
Low avidity may persist in some people

Method

ELISA/CLIA with urea dissociation step

Good to know

High avidity excludes recent infection reliably; low or borderline avidity alone does not prove it.

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