Clinical Information

Toxoplasma gondii is an intracellular protozoan parasite that infects humans and many animals; cats are the primary host. Human infection is most commonly acquired through ingestion of undercooked meat containing tissue cysts or exposure to oocysts from cat feces. Serologic testing for Toxoplasma antibodies is useful in the evaluation of suspected toxoplasmosis, especially in pregnant patients, immunocompromised individuals, and patients with compatible clinical findings.

In immunocompetent individuals, infection is often asymptomatic or causes a mild self-limited illness. When present, symptoms may include fever, lymphadenopathy, fatigue, myalgias, arthralgias, headache, pharyngitis, hepatosplenomegaly, or rash. Less commonly, toxoplasmosis may involve the eye, central nervous system, heart, lungs, liver, or skeletal muscle.

In pregnancy, maternal infection may be transmitted to the fetus. The risk of transmission increases with gestational age, while infection earlier in pregnancy is generally associated with more severe fetal disease. Congenital infection may result in serious complications, including chorioretinitis, intracranial calcifications, hydrocephalus or microcephaly, and other neurologic abnormalities.

In immunocompromised patients, including transplant recipients, patients receiving chemotherapy, and individuals with advanced immunosuppression, toxoplasmosis may present as severe primary infection or reactivation of latent infection. Clinical manifestations may include encephalitis, headache, confusion, fever, focal neurologic deficits, or ocular disease.

Test interpretation

Toxoplasma IgG

• Detection of Toxoplasma IgG indicates current or prior exposure to T. gondii. A positive IgG result alone does not distinguish recent infection from remote infection. In most cases, IgG persists long term after infection.
• A negative IgG result suggests no serologic evidence of prior exposure, although very early infection cannot be excluded.
• When recent infection is a concern, especially during pregnancy, IgG results should be interpreted together with IgM and, when appropriate, follow-up serology and confirmatory reference laboratory testing.

Recommended use

Toxoplasma serology should be ordered when there is clinical suspicion for toxoplasmosis or a specific need to assess prior exposure or possible recent infection. Testing is particularly relevant in:

• Pregnant patients with possible recent exposure or suspected infection
• Immunocompromised patients with suspected primary infection or reactivation
• Patients with compatible lymphadenopathy, ocular findings, or neurologic symptoms
• Evaluation of possible congenital infection in the appropriate clinical setting

Serologic testing should not be used as a general screening test in populations with low pretest probability.

Important limitations

• IgM results do not by themselves confirm recent infection.
• IgM antibodies may remain detectable for many months after infection.
• Positive and negative predictive values depend on the clinical setting and disease prevalence.
• Serologic results should always be interpreted in conjunction with clinical findings and other laboratory or imaging studies.
• Heterophilic antibodies or other assay interferences may affect immunoassay results.
• In immunocompromised patients, serologic responses may be atypical and additional testing such as molecular detection may be necessary.

Additional diagnostic considerations

When active toxoplasmosis is strongly suspected, especially in pregnancy, congenital infection, ocular disease, or central nervous system disease in immunocompromised patients, additional evaluation may include:

• Repeat serology on a follow-up specimen
• IgG avidity testing
• Molecular testing such as PCR on appropriate specimens
• Correlation with imaging, ophthalmologic findings, or tissue diagnosis when indicated