Test ID: TOXB
Toxoplasma gondii, Molecular Detection, PCR, Blood

Supporting the diagnosis of active toxoplasmosis, particularly in immunocompromised individuals

Toxoplasma gondiiis 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,T gondiican 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 toT 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.

Detection ofT gondiiDNA by PCR has proven to be a rapid and reliable alternative or supportive method for the diagnosis of toxoplasmosis. When performed on blood, it may detect circulating parasite DNA and thus confirm or support the results of serologic testing. PCR testing on peripheral blood has been used successfully to detect cases of ocular toxoplasmosis(2) as well as invasive disease in allogeneic stem cell recipients.(3,4) However, blood may not be a sensitive specimen for detecting organ specific disease (eg, ocular or cerebral toxoplasmosis). In this case, other specimens (eg, ocular fluid, CSF, fresh tissue) should be considered (order PTOX /Toxoplasma gondii,Molecular Detection, PCR).

Negative

A positive result indicates presence of DNA fromToxoplasma gondii.

Negative results indicate absence of detectable DNA, but do not exclude the presence of organism or active or recent disease.

This assay is designed for use in patients with a clinical history and symptoms consistent with toxoplasmosis. This test should not be used to screen healthy patients.

Blood may not be a sensitive specimen for detecting organ specific disease (eg, ocular or cerebral toxoplasmosis). In this case, other specimens (eg, ocular fluid, spinal fluid, fresh tissue) should be considered.

Results should be interpreted with consideration of clinical and laboratory findings. A negative result does not indicate absence of disease. Reliable results depend on adequate specimen collection and the absence of inhibiting substances.

Accuracy/Diagnostic Sensitivity and Specificity:

Accuracy was determined using a combination of spiking and clinical specimens for each source accepted for testing. Accuracy ranges from 97% to 100%.

Analytical Sensitivity/Limit of Detection (LoD):

The limit of detection for this assay is less than 5,000 copies/mL

Analytical Specificity:

No PCR signal was obtained from extracts of 20 bacterial, parasitic, and viral isolates from similar organisms and from organisms commonly found in the specimen types tested.

Precision:

Intra-assay precision and interassay precision are 100%.

Reference Range:

The reference range is "Negative" for this assay.

Reportable Range:

This is a qualitative assay and results are reported as "Negative" or "Positive."

1. Robert-Gangneux F, Darde M. Epidemiology of and diagnostic strategies for toxoplasmosis. Clin Microbiol Rev 2012;25(2):264

2. Mattos CCB, Meira CS, Ferreira AIC, et al: Contribution of laboratory methods in diagnosing clinically suspected ocular toxoplasmosis in Brazilian patients. Diagn Microbiol Infec Dis 2011;70:362-366

3. Martino R, Bretagne S, Einsele H, et al: Early detection of Toxoplasma infection by molecular monitoring of Toxoplasma gondii in peripheral blood samples after allogeneic stem cell transplantation. Clin Infect Dis 2005;40:67-78

4. Fricker-Hidalgo H, Bulabois C, Brenier-Pinchart M, et al: Diagnosis of toxoplasmosis after allogeneic stem cell transplantation: results of DNA detection and serological techniques. Clin Infect Dis 2009;48:e9-e15