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.
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.
Toxoplasma gondii tachyzoites, stained by
immunofluorescence (IFA). This is a positive reaction (tachyzoites + human antibodies to Toxoplasma + FITC-labelled
antihuman IgG = fluorescence.)
C: Negative IFA for antibodies to T. gondii, polar stain reaction.