Often a latex agglutination or ELISA assay identifying combined (IgG and IgM) antibodies.

Usually performed at the local hospital laboratory.

First Ig to be detected: appears within 1–2 weeks of exposure.

May be difficult to interpret because: (A) can remain detectable beyond 2 years after infection in some highly sensitive assays; (B) false positive rate as high as 2%; (C) transplacental leakage of maternal T. gondii-specific IgM can lead to low positive T. gondii-specific IgM in an uninfected infant shortly after birth (should disappear within 1–2 weeks); (D) only 50%–60% of congenitally infected babies have detectable levels of T. gondii-specific IgM (and IgA) in the first month of life; levels may be undetectable even in severe disease, if maternal seroconversion occurs early in pregnancy, or if treatment is given in pregnancy or at or soon after birth.

Consider congenital infection if T. gondii-specific IgM is present in infant and not in mother.

IgA can cross the placenta (usually disappearing by 2 weeks of life), therefore: (A) passive transfer must be excluded; (B) only consider a positive result significant if the mother is T. gondii-specific IgA negative.

May not be produced by congenitally infected infant in the first months of life.

Negative neonatal T. gondii-specific IgG essentially excludes congenital toxoplasmosis.

Appears from 2 weeks of primary infection and persists for life.

Maternal: seroconversion of IgM, IgG or IgA during pregnancy suggests recent acute maternal toxoplasma infection.

Neonatal: IgG crosses the placenta and therefore in the newborn may reflect past or current maternal infection.

Transplacental maternal T. gondii-specific IgG has a half-life of around 3–4 weeks. Persistence beyond 12 months of age is considered confirmatory of congenital infection.

Measures T. gondii-specific IgM and IgG antibody.

Particularly helpful if: (A) initial dye tests are significantly raised or serial dye test results demonstrate increasing levels over time; (B) infant dye test levels ≥4× maternal levels are suggestive of congenital infection.

Measures functional affinity of T. gondii-specific IgG antibody.

Useful when T. gondii-specific IgM samples are equivocal: may help discriminate between past (especially preconception) and recently acquired infection.

High avidity reliably excludes recent infection, but low avidity is not a reliable indicator of recent infection since rise in avidity may be delayed in some individuals.

Molecular diagnostic techniques, for example, PCR.

Can be done on amniotic fluid, blood or CSF samples.

Detection of T. gondii in amniotic fluid or infant specimens confirms congenital infection.