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Toxoplasma IgG negative and IgM positive in pregnancy

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CDC toxoplasmosis pregnancy positive IgM negative IgG confirmatory testing avidity

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toxoplasmosis pregnancy serology

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https://www.cdc.gov/dpdx/toxoplasmosis/index.html

Toxoplasma IgG negative + IgM positive in pregnancy is indeterminate. It does not prove a new infection.
Most commonly, this pattern is due to a false-positive IgM result. Less commonly, it can represent a very early infection, before IgG antibodies have appeared.
What to do promptly
  1. Contact your obstetrician or maternal-fetal medicine specialist today or next working day.
  2. Repeat Toxoplasma IgG and IgM in about 2 to 3 weeks, preferably through the same laboratory, and send the original and repeat samples for reference-laboratory confirmatory testing.
  3. If IgG becomes positive or rises on repeat testing, that supports a recent infection. A reference lab may use a more detailed antibody panel. IgG avidity is useful only once IgG is detectable.
  4. Do not start anti-toxoplasma treatment based on this one screening result alone. Management depends on whether recent maternal infection is confirmed and on gestational age.
Commercial Toxoplasma IgM tests have enough false-positive results that they should not be used alone to diagnose infection in pregnancy. Confirmation at a specialist reference laboratory is recommended. Medical Microbiology 9e, p. 823. The CDC diagnostic guidance similarly advises confirmatory testing when timing of infection is important in pregnancy.
Until clarified, use prevention measures: avoid undercooked meat, wash produce, wear gloves for soil/gardening, and avoid changing cat litter if possible. If you must do it, use gloves and wash hands well.
A recent systematic review also supports using combined repeat serology and additional assays rather than interpreting IgM alone (PMID 40329230).
Seek urgent assessment if you have fever, swollen neck glands, visual symptoms, or if your clinician believes you had a meaningful recent exposure.

How is toxoplasmosis confirmed when IgM is positive and IgG is negative?

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CDC toxoplasmosis IgM positive IgG negative repeat serology reference laboratory confirmation pregnancy

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toxoplasmosis pregnancy serology

With IgM positive and IgG negative, toxoplasmosis is not confirmed by that initial result. It may be a false-positive IgM or a very early infection.
Confirmation is done by:
  1. Repeat Toxoplasma IgG and IgM on a new blood sample after about 2 to 3 weeks.
    • IgG seroconversion, meaning IgG changes from negative to positive, confirms a newly acquired infection.
    • A significant rise in IgG titre in paired samples also supports acute infection.
  2. Send the original and follow-up samples to a specialist/reference laboratory for confirmatory serology.
    • They use a panel of more specific tests to verify whether IgM is truly Toxoplasma-specific and assess the pattern of recent versus older infection.
    • A positive commercial screening IgM by itself is insufficient, particularly in pregnancy.
  3. PCR for Toxoplasma DNA may confirm infection if there is an appropriate specimen and clinical indication.
    • For suspected fetal infection, amniotic-fluid PCR is the preferred diagnostic test, after discussion with maternal-fetal medicine. It is not the first step for an isolated maternal IgM-positive/IgG-negative result.
IgG avidity cannot help yet because there is no measurable IgG. It can be considered only after IgG becomes positive.
The best evidence of a true recent maternal infection in this situation is therefore documented IgG seroconversion on serial samples, together with reference-lab confirmation. Creasy & Resnik’s Maternal-Fetal Medicine, p. 1219. The CDC 2024 case definition likewise recognizes seroconversion during pregnancy or a fourfold IgG rise in samples taken at least 3 weeks apart as confirmatory evidence.
Arrange this promptly through your obstetrician or maternal-fetal medicine specialist.
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