Toxoplasma IgM and IgG (Toxoplasmosis)
Definition
Detects antibodies against the parasite Toxoplasma gondii. Serological testing is the routine diagnostic method; other methods are microscopic examination, detection of genetic material by PCR and (rarely) isolation of the parasite.
Purpose
Detection of toxoplasmosis infection, particularly in pregnant women and immunosuppressed individuals.
Results
- IgG positive, indicates that infection occurred at some point, in the past or currently
- IgM negative, largely rules out recent infection
- IgM positive, is problematic (see below)
Risks and Limitations
A positive IgM does not mean "new infection":
Toxoplasma-specific IgM antibodies can be detected for up to 18 months after a newly acquired infection.
In other words, a positive IgM may relate to an infection from a year and a half ago. In pregnancy it can be mistaken for a "new infection" and lead to unnecessary anxiety and intervention.
The sensitivity and specificity of these tests, particularly the IgM tests, can vary by method and by laboratory.
Recommendation: when determining the timing of infection is critical, for example in pregnant women, follow-up or confirmatory testing is recommended.