How Is Toxoplasmosis Diagnosed?

Toxoplasmosis ( Toxoplasma gondii ) has a complicated life cycle. Unlike most microorganisms, it is difficult to culture (grow in a laboratory; a procedure often used to identify infectious agents). Therefore, your doctor must rely on other means to make a positive diagnosis.

Serologic Testing

The most effective way to detect toxoplasmosis is to test the blood for specific antibodies against the parasite. (Antibodies are cells your immune system produces to combat specific infectious agents that have entered your body.) This process is called serologic testing, or serology. Unfortunately, blood tests for toxoplasma antibodies are occasionally associated with false-positive results (the test incorrectly diagnoses infection). If your test suggests infection, your doctor will probably have the results verified at a special reference laboratory before beginning treatment.

Two types of antibodies are usually detected during serologic testing. Approximately one to two weeks after infection, your immune system begins to produce the anti-toxo IgM antibody, which can be detected in your blood for six to 12 months before disappearing. (For unknown reasons, some people continue to have anti-toxo IgM antibodies in their blood for several years following infection.) Several weeks after the anti-toxo IgM antibody appears, your immune system begins to produce the anti-toxo IgG antibody. For the next several months, your immune system will continue to produce large amounts of the anti-toxo IgG antibody. After that, production and levels of the anti-toxo IgG antibody in the blood begin to decline; however, unlike the anti-toxo IgM antibody, anti-toxo IgG usually remains in your blood for life.

The following table highlights the probable infection status, depending on the absence or presence of these antibodies.

Table 1: Anti-toxoplasma Antibodies as Indicator of Infection History
Anti-toxoplasma IgMAnti-toxoplasma IgGToxoplasma Infection Status
AbsentAbsentno current or previous infection
PresentAbsentvery early infection
PresentPresentcurrent infection or infection in the previous 6-12 months
AbsentPresentprevious infection, probably more than 6-12 months ago

Unfortunately, serologic testing for toxoplasmosis is not as straightforward as this table suggests. Because tests screen for signs of the body's reaction to an infection-not the infection itself-they are not completely reliable. There is also a wide variation in the ability of specific laboratories to accurately detect anti-toxo IgM antibodies. Additionally, some individuals have anti-toxo IgM antibodies present for a few months following an infection, while others may have the antibody in their blood for over a year.

The most effective way to detect toxoplasmosis during pregnancy is to screen for anti-toxo IgG antibodies in a woman who did not previously have the antibodies. However, most people do not know if they ever had toxoplasmosis, and few women are tested before becoming pregnant.

It is also possible to distinguish between recent acute and chronic infections by performing special studies, called avidity tests, on the maternal anti-toxo IgG antibodies. These studies are usually performed at an established reference laboratory.

Ultrasound Testing

Ultrasound examinations allow your doctor to look at your baby in the womb. This way, she can screen for congenital toxoplasmosis, which may damage a baby's internal organs. However, not all infants of infected mothers develop congenital toxoplasmosis, so an ultrasound may not help diagnose infection in the mother. When screening for fetal infection, your doctor must also consider that not all infants look abnormal under ultrasound and symptoms may not appear until several weeks or months after the initial infection.