What is an IgG Avidity Test for Toxoplasmosis in Pregnancy?

An IgG avidity test is a specialized blood test used during pregnancy to help estimate exactly when you were infected with the Toxoplasma gondii parasite ^[1][2]. If a standard blood test shows you have toxoplasmosis antibodies, doctors use the avidity test to figure out if this is a brand new infection that happened during your pregnancy, or an old infection from months or years ago ^[1][3]. Distinguishing between a recent and past infection is crucial, because a recent infection during pregnancy carries a risk of being passed to your baby, while an older infection generally does not ^[4][5].

How the Avidity Test Works

When your body first fights off an infection, your immune system produces antibodies that are somewhat weak at grabbing onto the parasite. Over the course of several months, your immune system refines these antibodies, making them much better and stronger at binding to the invader ^[1][6].

Avidity simply refers to this binding strength—how tightly your antibodies hold onto the parasite ^[6]. By measuring this grip strength, the laboratory can estimate how long your body has been producing these antibodies.

What Your Results Mean

Your avidity test results will usually be categorized into one of three zones, which help guide your care:

• High Avidity: Your antibodies are binding very tightly to the parasite. This means the infection likely happened at least three to four months before the test was taken ^[7][8]. If you take this test early in your first trimester, a high avidity result is excellent news—it means you caught the infection before you got pregnant, and the risk to your baby is extremely low ^[3][7]. However, if you are tested later in your pregnancy (during your second or third trimester), a high avidity result means the infection could have happened three to four months ago—which might place the infection right at the beginning of your pregnancy ^[3]. In that case, your doctor will evaluate this result alongside other tests to understand your timeline.
• Low Avidity: Your antibodies are binding loosely. This suggests that the infection is recent, likely within the last few months ^[9]. While this means the infection may have occurred during your pregnancy and carries a higher risk, low avidity alone isn’t a perfect measure ^[9][10]. Some people continue to produce low-avidity antibodies for a long time, so doctors will always look at this result alongside other blood tests ^[10][11].
• Gray Zone or Borderline Avidity: Your results fall somewhere in the middle. These results are inconclusive, meaning it is unclear whether the infection is recent or from the past ^[8]. Because a recent infection cannot be definitively ruled out, your doctor will likely recommend repeating the blood test in three to four weeks to see if your antibody levels or avidity change ^[12][8].

What Happens Next?

Because interpreting these tests is complex, your blood sample is sent to a specialized clinical laboratory. Once the results return, your doctor will use them to determine the best next steps for you and your baby ^[2][13].

If your results show high avidity early in your first trimester, your doctor will usually offer reassurance, and you likely won’t need any special treatment or further invasive testing ^[7][3].

If your results show low or borderline avidity, your care team will monitor you more closely. They may:

• Prescribe medication: You may be started on an antibiotic like spiramycin. This medication is used to help prevent the parasite from crossing the placenta and reaching your baby ^[14][15]. If later testing confirms the baby has been infected, your medical team will likely switch your medication to a targeted therapy to treat the baby directly ^[14][15].
• Recommend an amniocentesis: To confirm whether the parasite has actually reached your baby, your doctor may suggest testing a small sample of your amniotic fluid ^[12][16]. This procedure is typically not performed immediately in early pregnancy; it is generally done after 18 weeks of pregnancy and at least four weeks after the suspected infection to ensure accurate results and safety ^[12][16].
• Run additional blood tests: Your doctor may combine the avidity test with other antibody tests (like IgM or IgA) to get a clearer picture of your infection timeline ^[11][13].