Jeroen Saeij

Toxoplasmosis

Blood tests explained: IgG, IgM and avidity

A toxoplasmosis test result is easy to misread, and misreadings have real consequences, including unnecessary treatment and unnecessary fear. This page explains what each part of the result means.

By Jeroen Saeij · Updated 17 September 2026

The short answer

IgG antibodies show that you have been infected at some point and now carry the parasite. IgM antibodies appear early in an infection, but they can linger for a year or more, so a positive IgM alone does not prove a recent infection. The avidity test resolves most of the doubt: high avidity means the infection is at least a few months old. In pregnancy, any result suggesting a recent infection deserves confirmation by a specialist laboratory before anyone acts on it.

What the tests measure

The parasite itself is hard to find in a person, so the tests look for the immune system’s response to it: antibodies in the blood1.

IgG antibodies appear within a couple of weeks of infection and then stay for life. A positive IgG means you have been infected and carry dormant cysts. It says nothing about when.

IgM antibodies also appear early. In textbooks they fade within months. In reality they can persist for a year or longer in some people, and the tests for them produce false positives. In the American national survey, about one person in a hundred was IgM positive, and among those who were positive for both IgM and IgG, almost all had high-avidity antibodies, meaning their infections were old2. A positive IgM, on its own, is a reason to look closer, not a diagnosis of recent infection.

Avidity measures how tightly the IgG antibodies bind the parasite. Antibodies mature over the months after infection, so low avidity fits a recent infection and high avidity does not. European studies established that high-avidity antibodies rule out an infection in the preceding three months or so3, and some test kits extend that window. Low avidity is weaker evidence in the other direction, because some people keep low-avidity antibodies for a long time.

Reading a result

IgG IgM What it usually means
Negative Negative Never infected. Susceptible, so precautions matter.
Positive Negative Infected in the past, typically more than six months ago. In pregnancy, reassuring.
Positive Positive Either a recent infection or a lingering IgM. Avidity and a reference laboratory decide.
Negative Positive Either the very start of an infection or a false positive. Repeat in two to three weeks.

Why the reference laboratory matters

The stakes of getting the timing wrong are highest in pregnancy, where “recent infection” can lead to treatment, invasive testing of the amniotic fluid, or worse. When a US reference laboratory looked at first-trimester samples flagged as possibly recent by ordinary tests, more than half turned out to have high-avidity antibodies, meaning the infection dated from before the pregnancy, and some of those women had already been started on treatment3. The laboratory in Palo Alto that did that work has served as the American reference laboratory for decades1. If your result suggests a recent infection and you are pregnant, ask for the sample to be sent there or to your country’s equivalent before decisions are made.

Tests for the baby

When a mother’s infection during pregnancy is confirmed, the question becomes whether the baby is infected. The main tool is a PCR test for parasite DNA in the amniotic fluid, done after about 18 weeks. After birth, the child’s own antibodies, ultrasound findings and eye examinations complete the picture4. All of this is specialist territory, and the decisions belong with an obstetrician and an infectious disease physician.

Can a test tell which strain you have?

This is my own corner of the field. Strains of the parasite differ in how dangerous they are, and the dangerous ones are concentrated in South America, so it would be useful to know which strain a person carries. Isolating the parasite from a patient is rarely possible, but antibodies carry a signature of the strain that raised them. Twenty years ago it was shown that short synthetic pieces of parasite proteins could distinguish the common European type from others in blood samples5. My lab extended the approach with larger arrays of peptides that separate the major lineages and flag the unusual strains that cause more severe disease67. These are research tools today, not something your doctor can order. The strains page explains why the question matters.

Sources

  1. Montoya JG. Laboratory diagnosis of Toxoplasma gondii infection and toxoplasmosis. J Infect Dis 2002;185 Suppl 1:S73-82. PubMed · DOI
  2. Jones JL, Kruszon-Moran D, Elder S et al. Toxoplasma gondii Infection in the United States, 2011-2014. Am J Trop Med Hyg 2018;98:551-557. PubMed · DOI
  3. Liesenfeld O, Montoya JG, Kinney S et al. Effect of testing for IgG avidity in the diagnosis of Toxoplasma gondii infection in pregnant women: experience in a US reference laboratory. J Infect Dis 2001;183:1248-53. PubMed · DOI
  4. Montoya JG, Remington JS. Management of Toxoplasma gondii infection during pregnancy. Clin Infect Dis 2008;47:554-66. PubMed · DOI
  5. Kong JT, Grigg ME, Uyetake L et al. Serotyping of Toxoplasma gondii infections in humans using synthetic peptides. J Infect Dis 2003;187:1484-95. PubMed · DOI
  6. Arranz-Solís D, Cordeiro C, Young LH et al. Serotyping of Toxoplasma gondii Infection Using Peptide Membrane Arrays. Front Cell Infect Microbiol 2019;9:408. PubMed · DOI
  7. Arranz-Solís D, Carvalheiro CG, Zhang ER et al. Toxoplasma GRA Peptide-Specific Serologic Fingerprints Discriminate Among Major Strains Causing Toxoplasmosis. Front Cell Infect Microbiol 2021;11:621738. PubMed · DOI