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Test ID: PTOX    
Toxoplasma gondii, Molecular Detection, PCR

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

Diagnosing central nervous system toxoplasmosis

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

Toxoplasma gondii is an intracellular protozoan parasite that chronically infects about 10% of the adult population in the United States. Transmission may occur by ingestion of undercooked meat containing cysts, by direct contact with the feces of an infected cat excreting infectious oocysts, and vertically through the placenta. Accurate diagnosis is crucial because of the different therapeutic options.

 

Central nervous system (CNS) toxoplasmosis most often occurs as a complication of HIV infection. Diagnosis of CNS toxoplasmosis is difficult by serologic methods, and direct detection of the organism in cerebrospinal fluid (CSF) by stain and culture is not sensitive. PCR detection of Toxoplasma gondii DNA in CSF is a rapid, sensitive, and specific method for the diagnosis of CNS toxoplasmosis.

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.

Negative for the presence of Toxoplasma gondii DNA

Interpretation Provides information to assist in interpretation of the test results

A positive result indicates presence of DNA from Toxoplasma gondii.

 

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

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

This assay is designed to detect only species of clinical significance and is to be used for patients with a clinical history and symptoms consistent with toxoplasmosis. This test should not be used to screen healthy patients. Depending on the population, varying percentages of patients may be found to be positive.

 

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.

Supportive Data

Accuracy/Diagnostic Sensitivity and Specificity:

The LightCycler real-time PCR method was compared to PCR-enzyme-linked immunosorbent assay (ELISA) for detecting the B1 gene of Toxoplasma gondii on 95 cerebrospinal fluid (CSF) clinical specimens received in the laboratory. Using the PCR-ELISA method as the gold standard, the diagnostic sensitivity and specificity of the assay was 82% (9 of 11 positives detected).

 

Supplemental Data (Spiking Studies):

To supplement the above data, 30 negative patient specimens (30 each: CSF, amniotic fluid, ocular fluid, and fresh tissue) were spiked with toxoplasma-positive control plasmid at the limit of detection (50 copies per microliter). The specimens were run in a blinded manner along with 30 of each specimen type negative (nonspiked) specimens; 97% to 100% of the spiked specimens were positive and 100% of the nonspiked specimens were negative.

 

Analytical Sensitivity/Limit of Detection (LoD):

The limit of detection for this assay was 50 copies/microliter in CSF, tissue, and body fluid (specifically ocular and amniotic fluid).

 

Analytical Specificity:

No PCR signal was obtained from extracts of 20 bacterial, parasitic, and viral isolates from similar organisms and from organisms (panel of 14 CSF with known viral and bacterial infections) commonly found in the specimen types tested.

 

Precision:

Intra-assay precision was 100% and inter-assay precision was 100%.

 

Reference Range:

A review of the literature indicates the reference range is "Negative" for this assay.

 

Reportable Range:

This is a qualitative assay and results are reported as "Negative" or "Positive for Toxoplasma gondii DNA." 

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

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

2. Sterkers Y, Varlet-Marie E, Marty P, et al: Diversity and evolution of methods and practices for the molecular diagnosis of congenital toxoplasmosis in France: a four years survey. Clin Microbiol Infect 2009 Nov 2

3. Villard O, Filisetti D, Roch-Deries F, et al: Comparison of enzyme-linked immunosorbent assay, immunoblotting, and PCR for diagnosis of toxoplasmic chorioretinitis. J Clin Microbiol 2003 Aug;41(8):3537-3541

4. 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 Sep;77(3):405-10