Toxoplasma gondii - Toxoplasmosis

Indications for Testing

Testing for toxoplasmosis should be considered in the following vulnerable groups:

• Pregnant individuals who are either immunocompromised or immunosuppressed, or who have symptoms of acute toxoplasmosis (ie, flu-like symptoms and blurred vision or photophobia)
• Immunocompromised individuals with symptoms of acute toxoplasmosis or reactivated disease (specifically, focal encephalitis that causes confusion, headache, motor weakness, and/or seizures)
• Neonates whose mothers experienced acute T. gondii infection during pregnancy

Laboratory Testing

Serology

Serology, using either indirect fluorescent antibody (IFA) tests or enzyme immunoassays (EIAs) such as enzyme-linked immunosorbent assays (ELISAs), is the most common diagnostic testing for toxoplasmosis. Both immunoglobulin G (IgG) antibodies, which indicate a past infection, and IgM antibodies, which indicate a more recent infection, may be assessed.  Interpretation of IgM results may be complicated because IgM antibodies lack specificity and may be detected up to 18 months after infection. 

If initial testing is not conclusive, additional testing (eg, IgA serology testing, IgE serology testing, IgG dye testing, IgG avidity testing, and differential agglutination testing) should be pursued at a specialized reference lab.  

Pregnant Individuals

If a pregnant individual tests positive for both IgG and IgM antibodies, an IgG avidity test should be performed to help differentiate between acute and past infection.  If performed within the first 12-16 weeks of pregnancy, the results should be interpreted as follows:

• High IgG avidity rules out T. gondii infection acquired during pregnancy. 
• Low IgG avidity does not rule out T. gondii infection acquired during pregnancy, but does not necessarily indicate recent infection. 

If a pregnant patient with suspected toxoplasmosis has low IgG titers, a second sample may be tested 2-3 weeks later. An increase in antibody titer suggests acute infection. 

If initial testing is not conclusive, samples should be sent to a specialized reference lab for further assessment.  For more information about interpreting serologic tests during pregnancy, see the Toxoplasmosis Serologic Testing for Pregnant Individuals algorithm.

Neonates

When congenital toxoplasmosis is suspected in neonates, IgA antibody levels should be assessed, in addition to IgM levels, because IgA antibody testing is more sensitive than IgM antibody testing in infants.  Because there are no FDA-approved commercial assays for IgA antibodies, testing should be performed at a specialized reference lab. 

Immunocompromised Individuals

Serologic testing is not always reliable in immunocompromised individuals; therefore, test results should be considered in the context of clinical presentation and other testing performed (eg, polymerase chain reaction  [PCR]). In patients with AIDS, for example, IgG antibodies are occasionally negative, and IgM antibodies are usually negative. 

Nucleic Acid Detection

PCR testing of amniotic fluid may be useful in diagnosing congenital T. gondii infection.  

PCR testing may also be useful in immunocompromised patients to assist in diagnosis, alongside serology and clinical presentation. However, a negative result does not rule out infection.  In patients with HIV, PCR testing of cerebrospinal fluid (CSF) may be considered to diagnose toxoplasmic encephalitis, which is caused by reactivated T. gondii infection, although the utility of this test is limited by low sensitivity. 

Immunohistochemistry

Immunohistochemistry (IHC) may also be used to diagnose T. gondii infection, although the use of IHC is limited because it may be difficult to obtain an appropriate specimen.