Cautions

Diagnosis of recent or active infection by Toxoplasma gondii can only be established based on a combination of clinical and serological data. The result of a single serum sample does not constitute sufficient proof for diagnosis of recent infection. Elevated IgM can persist from an acute infection that may have occurred as long ago as 1 year.

To differentiate between a recently acquired and past infection in patients who are IgM and IgG positive for Toxoplasma antibodies, Toxoplasma IgG avidity testing should be considered. A high avidity index for IgG antibodies indicates that the infection occurred at least 4 months ago. No clinical interpretation can be deduced from a low avidity result.

A negative Toxoplasma IgM result in combination with a positive IgG result does not completely rule out the possibility of an acute infection with Toxoplasma.

Elevated anti-IgM or IgG titers may be absent in patients who are immunocompromised. Results should be interpreted with caution in patients who are either HIV-positive, receiving immunosuppressive therapy, or have other disorders leading to immunosuppression.

A suspected diagnosis of central nervous system or congenital toxoplasmosis should be confirmed by detection of T gondii DNA by polymerase chain reaction (PCR) analysis of cerebrospinal fluid or amniotic fluid specimens, respectively (PTOX / Toxoplasma gondii, Molecular Detection, PCR, Varies).

If a serum specimen was collected too soon after infection, antibodies to T gondii may be absent. If this is suspected, a second serum specimen should be collected 2 to 3 weeks later, and the test repeated.

Heterophile antibodies in the patient specimens may interfere with IgM assay performance.

The performance of these assays has not been established for cord blood testing.

Specimens should not be collected from patients receiving therapy with high biotin doses (ie, >5 mg/day) until at least 8 hours following the last biotin administration.

As with any low prevalence analyte, there is the increased possibility that a positive result may be false, reducing the assay's positive predictive value. Per the Public Health Advisory (7/25/1997), the US Food and Drug Administration suggests that sera found to be positive for T gondii IgM antibodies should be submitted to a Toxoplasma reference laboratory.

The anti-Toxoplasma IgG/IgM results in a given specimen, as determined by assays from different manufacturers, can vary due to differences in assay and reagent methods. Results from assays of other manufacturers cannot be used interchangeably.