Table 1

Interpretation of common laboratory tests (maternal and infant) in suspected toxoplasmosis6 7 9–12

Laboratory testSignificance
Screening test for presence of T. gondii-specific antibodies
  • Often a latex agglutination or ELISA assay identifying combined (IgG and IgM) antibodies.

  • Usually performed at the local hospital laboratory.

T.gondii-specific IgM
  • 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.

T.gondii-specific IgA
  • 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.

T.gondii-specific IgG
  • 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.

Sabin-Feldman dye test
  • 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.

Avidity test
  • 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.

T.gondii-specific NAAT
  • 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.

  • CSF, cerebrospinal fluid; Ig, immunoglobulin; NAAT, nucleic acid amplification test; PCR, Polymerase chain reaction; T.gondii, Toxoplasma gondii.