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Test ID: TOXOP    
Toxoplasma gondii Antibody, IgM and IgG (Separate Determinations), Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Determining whether a patient has had previous exposure to or recent infection with Toxoplasma gondii

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Toxoplasma gondii is an obligate intracellular protozoan parasite that is capable of infecting a variety of intermediate hosts including humans. Infected definitive hosts (cats) shed oocysts in feces that rapidly mature in the soil and become infectious.(1) Toxoplasmosis is acquired by humans through ingestion of food or water contaminated with cat feces or through eating undercooked meat containing viable oocysts. Vertical transmission of the parasite through the placenta can also occur, leading to congenital toxoplasmosis. Following primary infection, Toxoplasma gondii can remain latent for the life of the host; the risk for reactivation is highest among immunosuppressed individuals.

 

Seroprevalence studies performed in the United States indicate that approximately 9% to 11% of individuals between the ages of 6 and 49 have antibodies to Toxoplasma gondii.(2)

 

Infection of immunocompetent adults is typically asymptomatic. In symptomatic cases, patients most commonly present with lymphadenopathy and other nonspecific constitutional symptoms, making definitive diagnosis difficult to determine.

 

Severe-to-fatal infections can occur among patients with AIDS or individuals who are otherwise immunosuppressed. These infections are thought to be caused by reactivation of latent infections and commonly involved the central nervous system.(3)

 

Transplacental transmission of the parasites resulting in congenital toxoplasmosis can occur during the acute phase of acquired maternal infection. The risk of fetal infection is a function of the time at which acute maternal infection occurs during gestation.(4) The incidence of congenital toxoplasmosis increases as pregnancy progresses; conversely, the severity of congenital toxoplasmosis is greatest when maternal infection is acquired early during pregnancy. A majority of infants infected in utero are asymptomatic at birth, particularly if maternal infection occurs during the third trimester, with sequelae appearing later in life. Congenital toxoplasmosis results in severe generalized or neurologic disease in about 20% to 30% of the infants infected in utero; approximately 10% exhibit ocular involvement only and the remainder are asymptomatic at birth. Subclinical infection may result in premature delivery and subsequent neurologic, intellectual, and audiologic defects.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Toxoplasma ANTIBODY, IgM

Negative

 

Toxoplasma IgM VALUE

<0.55 (Negative)

0.55 to <0.65 (Equivocal)

> or =0.65 (Positive)

 

Toxoplasma ANTIBODY, IgG

Negative

 

Toxoplasma IgG Value

< or =9 IU/mL (Negative)

10-11 IU/mL (Equivocal)

> or =12 IU/mL (Positive)

Interpretation Provides information to assist in interpretation of the test results

Active toxoplasmosis is suggested by the presence of IgM antibodies, but elevated anti-IgM titers are often absent in immunocompromised patients. In addition, elevated IgM can persist from an acute infection that may have occurred as long ago as 1 year.

 

IgG is only indicative of previous exposure to Toxoplasma (recent or past). A single positive Toxoplasma IgG result should not be used to diagnose recent infection. Seroconversion from negative to positive IgG is indicative of recent Toxoplasma gondii infection.

 

A suspected diagnosis of acute toxoplasmosis should be confirmed by detection of Toxoplasma gondii DNA by PCR analysis of cerebrospinal fluid or amniotic fluid specimens (PTOX / Toxoplasma gondii, Molecular Detection, PCR).

 

For further confirmation of a diagnosis, the FDA issued a Public Health Advisory (7/25/1997) suggesting that sera found to be positive/equivocal for Toxoplasma gondii IgM antibody be sent to a Toxoplasma reference laboratory. Recommended laboratories included the CDC or Jack Remington MD, Palo Alto Medical Foundation, 860 Bryant St., Palo Alto, CA 94301.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Sera drawn very early during the acute stage of infection may have Toxoplasma IgG levels <9 IU/mL.

 

The Toxoplasma IgG assay should not be used alone to diagnose recent Toxoplasma gondii infection. Results should be considered in conjunction with clinical presentation, patient history, and other laboratory findings.

 

IgG is not useful for diagnosing infection in infants <6 months of age. IgG antibodies in that age group usually are the result of passive transfer from the mother.

 

The performance characteristics of this assay have not been evaluated in immunocompromised individuals and have not been established for cord blood or for testing of neonates.

Supportive Data

IgM:

The VIDAS TOXO IgM system was compared to an indirect immunofluorescence (IFA) IgM assay (GenBio, San Diego, CA). Of 125 specimens tested, 45 and 47 were IFA and VIDAS positive, respectively. Using the IFA IgM as the gold standard, the sensitivity and specificity of the VIDAS Toxo IgM assay was 98% and 96%, respectively.

 

IgG:

To evaluate the accuracy of the BioPlex Toxoplasma IgG multiplex flow immunoassay, 600 prospective serum samples submitted for routine Toxoplasma IgG testing by the VIDAS enzyme linked-fluorescence immunoassay (ELFA; bioMerieux, Durham, NC) were also analyzed in a blinded fashion by the BioPlex assay within a 24-hour period. Samples with discordant results after initial testing were repeated by both assays during the same freeze/thaw cycle. Further resolution of discrepant results was performed by sending the samples to the Palo Alto medical Foundation for testing. The results are summarized below:

 

Toxoplasma IgG (VIDAS ELFA)

BioPlex Toxoplasma IgG

 

Positive

Negative

Equivocal

Positive

63

2(a)

6

Negative

0

528

0

Equivocal

0

0

1

a. Both of these serum samples were negative by the Sabin-Feldman dye test at the Palo Alto Medical Foundation Toxoplasma laboratory.

Sensitivity: 100% (63/63); 95% Confidence Interval (95% CI): 93.1%-100%

Specificity: 99.6% (528/530); 95% CI: 98.5%-99.9%

Overall Percent Agreement: 98.7% (592/600); 95% CI: 97.3%-99.4%

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Tenter AM, HeckerothAR, Weiss LM: Toxoplasma gondii: from animals to humans. Int J Parasitol 2000;30(12-13):1217

2. Jones JL, Kruszon-Moran D, Sanders-Lewis K, Wilson M: Toxoplasma gondii infection in the United States, 1999–2004, decline from the prior decade. Am J Trop Med Hyg 2007;77(3):405

3. Luft BJ, Remigton JS: Toxoplasmic encephalitis in AIDS. Clin Infect Dis 1992;15(2):211-222

4. Wong SY, Remington JS: Toxoplasmosis in pregnancy. Clin Infect Dis 1994;18(6):853-862