DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/toxoplasmosis.
The diagnosis of toxoplasmosis may be documented by:
- Observation of parasites in patient specimens, such as bronchoalveolar lavage material from immunocompromised patients, or lymph node biopsy.
- Isolation of parasites from blood or other body fluids, by intraperitoneal inoculation into mice or tissue culture. The mice should be tested for the presence of Toxoplasma organisms in the peritoneal fluid 6 to 10 days post inoculation; if no organisms are found, serology can be performed on the animals 4 to 6 weeks post inoculation.
- Detection of parasite genetic material by PCR, especially in detecting congenital infections in utero.
- Serologic testing is the routine method of diagnosis.
The detection of Toxoplasma-specific antibodies is the primary diagnostic method to determine infection with Toxoplasma. Toxoplasma antibody detection tests are performed by a large number of laboratories with commercially available kits.
An algorithm for the immunodiagnosis of toxoplasmosis for individuals greater than one year of age is shown table below. The IFA and EIA tests for IgG and IgM antibodies are the tests most commonly used today. Persons should be initially tested for the presence of Toxoplasma-specific IgG antibodies to determine their immune status. A positive IgG titer indicates infection with the organism at some time. If more precise knowledge of the time of infection is necessary, then an IgG positive person should have an IgM test performed by a procedure with minimal nonspecific reactions, such as IgM-capture EIA. A negative IgM test essentially excludes recent infection, but a positive IgM test is difficult to interpret because Toxoplasma-specific IgM antibodies may be detected by EIA for as long as 18 months after acute acquired infection.
A major problem with Toxoplasma-specific IgM testing is lack of specificity. Two situations occur frequently: i) persons with a positive IgM but negative IgG, and ii) individuals with positive IgG and IgM results. In the first situation, a positive IgM result with a negative IgG result in the same specimen should be viewed with great suspicion; the patient's blood should be redrawn two weeks after the first and tested together with the first specimen. If the first specimen was drawn very early after infection, the patient should have highly positive IgG and IgM antibodies in the second sample. If the IgG is negative and the IgM is positive in both specimens, the IgM result should be considered to be a false positive and the patient should be considered to be not infected. In the second situation, a second specimen should be drawn and both specimens submitted together to a reference lab which employs a different IgM testing system for confirmation. Prior to initiation of patient management for acute toxoplasmosis, all IgG/IgM positives should be submitted to a reference lab for IgG avidity testing.
If the patient is pregnant, and IgG/IgM positive, an IgG avidity test should be performed. A high avidity result in the first 12 to 16 weeks of pregnancy (time dependent upon the commercial test kit) essentially rules out an infection acquired during gestation. A low IgG avidity result should not be interpreted as indicating recent infection, because some individuals have persistent low IgG avidity for many months after infection. Suspected recent infection in a pregnant woman should be confirmed prior to intervention by having samples tested at a toxoplasmosis reference laboratory. If the patient has clinical illness compatible with toxoplasmosis but the IgG titer is low, a follow-up titer two to three weeks later should show an increase in antibody titer if the illness is due to acute toxoplasmosis, assuming the host is not severely immunocompromised.
|IgG Result||IgM Result||Report/interpretation for humans*|
|Negative||Negative||No seroiogical evidence of infection with Toxoplasma.|
|Negative||Equivocal||Possible early acute infection or false-positive IgM reaction. Obtain a new specimen for IgG and IgM testing. If results for the second specimen remain the same, the patient is probably not infected with Toxoplasma.|
|Negative||Positive||Possible acute infection or false-positive IgM result. Obtain a new specimen for IgG and IgM testing. If results for the second specimen remain the same, the IgM reaction is probably a false-positive.|
|Equivocal||Negative||Indeterminate: obtain a new specimen for testing or retest this specimen for IgG In a different essay.|
|Equivocal||Equivocal||Indeterminate: obtain a new specimen for both IgG and IgM testing.|
|Equivocal||Positive||Possible acute infection with Toxoplasma. Obtain a new specimen for IgG and IgM testing. If results for the second specimen remain the same or if the IgG becomes positive, both specimens should be sent to a reference laboratory with experience in diagnosis of toxopiasmosis for further testing.|
|Positive||Negative||Infected with Toxoplasma for more than 1 year.|
|Positive||Equivocal||Infected with Toxoplasma for probably more than 1 year or false-positive IgM reaction. Obtain a new specimen for IgM testing. If results with the second specimen remain the same, both specimens should be sent to a reference laboratory with experience in the diagnosis of toxopiasmosis for further testing.|
|Positive||Positive||Possible recent infection within the last 12 months, or false-positive IgM reaction. Send the specimen to a reference laboratory with experience in the diagnosis of toxopiasmosis for further testing.|
- *except infants
Newborn infants suspected of congenital toxoplasmosis should be tested by both an IgM- and an IgA-capture EIA. Detection of Toxoplasma-specific IgA antibodies is more sensitive than IgM detection in congenitally infected babies. None of the current commercial assays offered in the United States have been cleared by the Food and Drug Administration for in vitro diagnostic use for infants; consequently, all specimens from neonates suspected of having congenital toxoplasmosis should be sent to the Toxoplasma Serology Laboratory, Palo Alto, CA which has the most experience with infant testing.
Serological determination of active central nervous system toxoplasmosis in immunocompromised patients is not possible at this time. Toxoplasma-specific IgG antibody levels in AIDS patients often are low to moderate, but occasionally no specific IgG antibodies can be detected. Tests for IgM antibodies are generally negative.
Several commercial kits for Toxoplasma serologic testing are available. However, the sensitivity and specificity of these kits may vary widely from one commercial brand to another. This is of concern because serology results can influence decisions on continuation or termination of pregnancies.
- NCCLS. Clinical Use and Intrepretation of Serologic Tests for Toxoplasma gondii; Approved Guideline. NCCLS document M36-A [ISBN 1-56238-523-2]. NCCLS, 940 West Valley Road, Suite 1400, Wayne, PA 19087-1898 USA, 2004.
- Wilson M, Jones JL, McAuley JM. Toxoplasma. In: Murray PR, Baron EJ, Pfaller MA, Jorgensen JH, Yolken RH, editors. Manual of Clinical Microbiology. 8th ed. Washington, D.C.: American Society for Microbiology; 2003. p. 1970-1980.
- Remington JS, McLeod R, Thulliez P, Desmonts G. Toxoplasmosis. In: Remington JS, Klein JO, editors. Infectious Diseases of the Fetus and Newborn Infant. 5th ed. Philadelphia, PA: The WB Saunders Co.; 2001. p. 205-346.
CDC 1998 Toxoplasma Human Serum Panel
To help evaluate the accuracy of commercial Toxoplasma antibody test kits, CDC created a Toxoplasma serum panel, the 'CDC 1998 Toxoplasma Human Serum Panel,' that contains known positive and negative sera. FDA now requires that any new commercial Toxoplasma test kit performs adequately based on results obtained using this panel. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr4902a5.htm
A panel of 100 human specimens (1.0 ml each) has been assembled to include samples which are Toxoplasma antibody negative, Toxoplasma-specific IgG positive but IgM negative, Toxoplasma-specific IgG and IgM positive and Toxoplasma-specific IgG and IgA positive. Specimens with varying levels of reactivity were included. The panel was tested by the Toxoplasma Serology Laboratory, Palo Alto Research Institute, Palo Alto, CA, for IgG, IgM, and IgA by the Sabin-Feldman Dye Test, the IgM-ELISA, the IgA-ELISA, and the differential agglutination (AC/HS) tests. The panel has also been tested at the Centers for Disease Control and Prevention (CDC) by IFA-IgG and the CDC EIA-IgM.
This panel is available for purchase from CDC by device manufacturers / U.S. distributors for Toxoplasma human antibody detection assays. After the panel is tested with a device, the results will be sent to CDC for analysis. The submitter will receive a written analysis from CDC including sensitivity and specificity rates for each immunoglobulin subclass tested, limited results of IgG reproducibility, graphs of device positive results vs. standard tests for each immunoglobulin subclass tested, and information regarding IgM and IgA results vs. time post-infection for both the device and the standard tests.
If you wish to purchase the panel, please mail or fax a letter on company letterhead stating that you wish to obtain the 'CDC Toxoplasma 1998 Human Serum Panel' to:
Division of Parasitic Diseases
Centers for Disease Control
1600 Clifton Road, Mailstop D-64
Atlanta, GA 30333