A toxoplasmosis blood test looks for antibodies against Toxoplasma gondii, a common parasite that most people who catch it never notice. The test rarely aims to explain current symptoms. Far more often it answers a different question: has your immune system already met this parasite, and if so, was that recently or long ago?
That distinction matters most in pregnancy and in people whose immune systems are suppressed. In this article you will learn what the two main antibody types mean, why a positive IgM result is so often misleading, how avidity testing resolves the ambiguity, and what recent research says about treatment and long-term follow-up.
What a toxoplasmosis blood test measures
Toxoplasma gondii is a single-celled parasite that completes its life cycle in cats but can infect almost any warm-blooded animal, including people. Humans usually acquire it by eating undercooked meat, by swallowing contaminated soil or water, or through contact with cat feces. Once inside the body it forms dormant cysts, mainly in muscle and brain tissue, where it stays for life without causing trouble in a healthy person.
Because the parasite is hard to find directly, laboratories look for the antibodies your immune system produced against it. The two that appear on almost every report are IgM and IgG. They rise on different schedules, and that timing is what the test exploits.
The sample is an ordinary blood draw with no fasting requirement. Results are usually reported as positive, negative or equivocal, sometimes with a numeric index in international units per milliliter.
The two antibodies and what they signal
| Antibody | When it appears | How long it lasts | What it suggests |
|---|---|---|---|
| IgM | Within one to two weeks of infection | Usually months, but sometimes years | Possible recent infection, but unreliable on its own |
| IgG | From about two weeks, peaking over a few months | Lifelong | Past exposure and lasting immunity |
The general behavior of these antibody classes is the same across many infections, and our IgG blood test guide and IgM results explainer cover that pattern in more detail.
Why your doctor ordered this test
- Pregnancy, either as routine screening in some settings or because of an abnormal ultrasound finding or exposure risk.
- A weakened immune system, for example after an organ transplant, during chemotherapy, or with advanced HIV infection. Our HIV test results guide explains the related screening.
- Unexplained swollen lymph nodes, prolonged fatigue or a mononucleosis-like illness.
- Eye inflammation affecting the retina, which toxoplasmosis can cause years after the original infection.
- Assessment of a newborn when infection during pregnancy is suspected.
- Before certain transplants, to match donor and recipient immune status.
In healthy adults with no symptoms, this test is not part of routine bloods. Around one in ten to one in three American adults carry antibodies without ever having been unwell, so a positive IgG in that context is a normal finding rather than a diagnosis.
How to read your toxoplasmosis result
| IgG | IgM | Usual interpretation | What follows |
|---|---|---|---|
| Negative | Negative | No evidence of infection, past or present, and no immunity | Prevention advice; repeat testing during pregnancy in some settings |
| Positive | Negative | Infection acquired in the past, now dormant | Usually reassuring; no action in a healthy person |
| Negative | Positive | Very early infection, or more often a false positive | Repeat both in two to three weeks to see whether IgG appears |
| Positive | Positive | Ambiguous; could be recent or long past | IgG avidity testing, often at a reference laboratory |
The bottom row is where most of the worry lives, and it is also where the extra test does its work.
What avidity testing adds
Avidity means how tightly an antibody grips its target. Early after an infection the immune system produces antibodies that bind loosely; over the following months it refines them until they bind firmly. Measuring avidity therefore dates the infection without needing a second sample weeks later.
High avidity indicates the infection happened at least three to five months ago. In a pregnancy in the first trimester, that effectively rules out infection during the pregnancy. Low or borderline avidity does not prove recent infection, because some people keep low-avidity antibodies for a long time, so it usually prompts further testing rather than immediate conclusions.
What the result means in pregnancy
The concern in pregnancy is not the mother’s health but transmission to the fetus, which can happen only if the infection is acquired during or shortly before the pregnancy. A woman who was infected years earlier is generally protected, and her baby is not at risk from that old infection.
Risk runs in two directions across the pregnancy. Transmission is least likely in the first trimester but causes the most severe effects when it happens; it is most likely in the third trimester but usually causes milder or delayed effects. This is why timing the infection is so central, and why a positive IgM alone is never enough to act on.
Practical prevention is straightforward and worth following if your IgG is negative: cook meat thoroughly, wash fruit, vegetables and hands after handling soil, wear gloves for gardening, and have someone else change the cat litter daily, since the parasite needs a day or more in the litter to become infectious.
When to see a doctor
- Contact your obstetric team promptly if you are pregnant and any part of your result is positive, so avidity or reference laboratory testing can be arranged without delay.
- Seek urgent care if you have a weakened immune system and develop headache, confusion, seizures, weakness on one side or changes in vision, since reactivation can affect the brain and eyes.
- Arrange a routine eye examination if you have known past infection and notice floaters, blurring or eye pain.
- No follow-up is needed for a positive IgG with negative IgM in a healthy, non-pregnant adult. That combination describes a common, dormant, harmless state.
If swollen glands and fatigue were the reason for testing, your clinician will usually also review a complete blood count, your lymphocyte levels and inflammation markers such as CRP results, since several infections produce a similar picture.
Latest scientific advances
Research over the past three years has focused on making these results easier to act on, particularly in pregnancy. Here is what it found, in everyday language.
IgM alone is not enough, and avidity fixes much of the problem
A 2025 systematic review pooled sixty-seven studies looking at IgM, IgG avidity and IgA testing in pregnant women. It found that a positive IgM on its own is often misleading, and that adding the avidity test substantially improves the ability to tell a recent infection from an old one. What this means for you is direct: a positive IgM in pregnancy should trigger an avidity test rather than immediate alarm, and a high-avidity result points to an old, harmless infection.
Treating during pregnancy makes a large difference
A 2025 systematic review covering fifty-six studies and more than eleven thousand pregnancies compared babies born to treated and untreated mothers. Infection reached the baby far less often when the mother was treated, roughly eighteen percent of the time compared with fifty-eight percent without treatment. These are pooled figures from studies of differing quality rather than a single controlled trial, so the exact numbers should be read as an indication rather than a precise prediction. The practical message is clear: if serology shows infection during pregnancy, starting treatment promptly substantially lowers the risk to the baby.
Children need eye checks for many years
A 2024 study followed six hundred and forty-six French children infected before birth for up to thirty-five years. About three in ten developed a lesion at the back of the eye, often appearing first around ages seven and twelve, though severe damage was uncommon. For families this reframes follow-up: a child diagnosed with congenital toxoplasmosis needs regular eye examinations well into adulthood, even when newborn testing and treatment went smoothly.
Glossary
| Term | Definition |
|---|---|
| Toxoplasma gondii | The single-celled parasite that causes toxoplasmosis. Cats are its definitive host. |
| IgM | The antibody class produced first after an infection. It fades over months, though sometimes it lingers much longer. |
| IgG | The longer-lasting antibody class that usually persists for life and indicates past exposure. |
| Avidity | How tightly an antibody binds its target. It increases over months, so it helps date an infection. |
| Seroconversion | The change from having no antibodies to having them, showing that infection occurred in between. |
| Congenital infection | An infection passed from a pregnant person to the baby before birth. |
| Equivocal | A result too close to the cutoff to be called positive or negative, usually prompting a repeat test. |
| Immunocompromised | Having a weakened immune system through illness or medication. |
| Retinochoroiditis | Inflammation at the back of the eye, the most common long-term complication of this infection. |
Frequently asked questions
What does a positive toxoplasmosis IgG mean?
It means you were infected at some point in the past and your immune system retains a lasting memory of it. In a healthy, non-pregnant adult this is common and generally reassuring, since it usually indicates protection against a new infection. The parasite stays dormant in the body for life, but it causes no problems unless the immune system becomes severely weakened. No treatment is given for a positive IgG alone.
Should I worry about a positive IgM result?
Not immediately. IgM is the antibody that appears first after infection, but it can persist for years and it is also prone to false positives, so a positive result on its own is unreliable. Your clinician will look at the IgG result alongside it, and in pregnancy will usually arrange an avidity test to work out when the infection happened. Many positive IgM results turn out to reflect an old infection or no infection at all.
What if both IgG and IgM are negative in pregnancy?
It means you have never been infected and have no immunity, so prevention matters. Cook meat thoroughly, wash produce and your hands after gardening or handling soil, wear gloves outdoors, and have someone else change the cat litter daily. Some care settings repeat the test later in pregnancy to catch a new infection. Being negative is not a problem in itself; it simply means the precautions apply to you.
Do I have to give up my cat?
No. Cats spread the parasite only briefly, usually for a couple of weeks after their own first infection, and the material in the litter tray needs a day or more before it becomes infectious. Changing the litter daily, ideally with someone else doing it during pregnancy, and washing hands afterward removes almost all the risk. Indoor cats fed commercial food are very unlikely to be shedding the parasite at all.
How is toxoplasmosis treated?
Healthy adults with no symptoms are not treated, because the dormant infection causes no harm. Treatment with specific antiparasitic medications is used during pregnancy when infection is confirmed or strongly suspected, in newborns with congenital infection, in eye disease affecting vision, and in people with weakened immune systems. The choice and duration of treatment depend on the situation and are decided by a specialist.
Can toxoplasmosis come back years later?
The dormant cysts can reactivate, but essentially only when the immune system becomes severely weakened, for instance during chemotherapy, after a transplant or with advanced HIV infection. In that setting reactivation can affect the brain or the eyes and needs urgent treatment. Separately, eye lesions can appear or recur years after a congenital infection, which is why long-term eye checks are recommended for those children.
Sources
- MedlinePlus, National Library of Medicine — Toxoplasma Blood Test — MedlinePlus Medical Encyclopedia, reviewed 2025 — medlineplus.gov
- Centers for Disease Control and Prevention — Clinical Overview of Toxoplasmosis — CDC, reviewed 2024 — cdc.gov
- Cleveland Clinic — Toxoplasmosis: Causes, Symptoms, Treatment and Prevention — Cleveland Clinic Health Library, reviewed 2026 — my.clevelandclinic.org
- Tork M, Sadeghi M, et al — Assessment of Simultaneous IgM, IgG Avidity, and IgA Testing in Diagnosis of Acute Toxoplasmosis in Pregnant Women: A Systematic Review and Meta-Analysis — BMC Pregnancy and Childbirth, 2025 — doi.org/10.1186/s12884-025-07580-6
- Ribeiro SK, Mariano IM, et al — Treatment Protocols for Gestational and Congenital Toxoplasmosis: A Systematic Review and Meta-Analysis — Microorganisms, 2025 — doi.org/10.3390/microorganisms13040723
- Journé A, Garweg J, et al — Long-Term Ocular Outcomes in Congenital Toxoplasmosis Treated Perinatally — Pediatrics, 2024 — doi.org/10.1542/peds.2023-064114
Further reading
- Understand the other antibody screen commonly run in pregnancy by reading our RPR blood test guide.
- Learn how lasting antibody memory is reported with our IgG blood test guide.
- Review the antibody class that signals recent infection in our IgM results explainer.
- Check the white cells that respond to parasitic and viral infection using our lymphocyte count guide.
- Explore the screening test for immune suppression with our HIV test results guide.
Understand your lab results with BloodSense
Two antibody lines and a word like equivocal can turn a routine prenatal panel into a stressful evening. BloodSense reads your full report and explains, in plain language, what IgG and IgM each indicate, how they combine, and which results genuinely call for a follow-up conversation. It helps you understand the pattern before your appointment. It does not diagnose infections and does not replace your obstetrician or family doctor.
