Toxoplasmosis Blood Test: IgG, IgM & Pregnancy

The toxoplasmosis blood test looks for antibodies against a parasite called Toxoplasma gondii. Its main job is to tell whether you are already immune (protected) or not immune — information that matters most during pregnancy, when a first infection can reach the baby. Toxoplasmosis is usually mild and often goes unnoticed; the real concern is catching a new (primary) infection at the wrong time. This guide explains how IgG, IgM, and the IgG avidity test are read, what each result pattern means, why the United States takes a different approach from countries like France, and how to lower your risk — without alarm. This test is part of the infection and serology tests often ordered in early pregnancy.

Key takeaways

• Toxoplasmosis is caused by the parasite Toxoplasma gondii; you catch it from undercooked meat, unwashed fruits and vegetables, or contact with contaminated soil or cat litter.¹²
• In a healthy person the infection is usually mild or silent; the risks concern mainly a primary infection during pregnancy (congenital toxoplasmosis) and people who are immunocompromised.³⁴
• The test measures two antibodies: IgG (older, generally protective immunity) and IgM (may flag a recent infection, but often stays positive for a long time and gives false positives).⁵⁶
• The IgG avidity test settles the question: high avidity early in pregnancy points to an old infection (more than 3–4 months), which is reassuring for the baby.⁶⁷
• Unlike France, the United States does not screen every pregnancy for toxoplasmosis; testing is targeted, and prevention is emphasized instead.¹⁸
• A positive IgM alone does not mean a recent infection or danger to the baby: it is the avidity test and follow-up with your clinician that decide.⁶⁸

What is toxoplasmosis?

Toxoplasmosis is an infection caused by a microscopic parasite, Toxoplasma gondii. Cats (and other felines) are its definitive host: the parasite reproduces in the cat's intestine and is shed in the feces. People are infected mainly in three ways: by eating undercooked meat (pork, lamb, beef, game) containing tissue cysts, by swallowing unwashed fruits, vegetables, or herbs contaminated with soil, or through contact with soil or an infected cat's litter box (gardening, cleaning litter).¹²

In a person with a normal immune system, toxoplasmosis is usually harmless: it is silent in most cases, or causes vague, mild symptoms (fatigue, low-grade fever, swollen lymph nodes in the neck, muscle aches) that clear on their own. Once the infection has passed, the body keeps lifelong immunity — which is why people generally get it only once.⁵⁴

Two situations deserve attention. First, a primary infection during pregnancy, because the parasite can then cross to the fetus (congenital toxoplasmosis). Second, a person who is immunocompromised (transplant, advanced HIV, chemotherapy), in whom an old infection can reactivate, often in the brain. Outside these settings, toxoplasmosis is rarely a concern.³¹

Why the test is done

The test answers one simple but crucial question: are you already immune, or not? Your clinician may order it mainly in these situations:²⁸

• Before or during pregnancy, when there is a reason to check your status — for example symptoms, a known exposure, or an abnormal ultrasound finding — to know whether you are protected;
• For suggestive symptoms: persistent swollen lymph nodes or a prolonged unexplained fever, especially in a pregnant woman;
• In an immunocompromised person: to establish status before a transplant or treatment and prevent reactivation;
• In a newborn, when maternal infection is suspected, to look for congenital toxoplasmosis.

Outside pregnancy and immunocompromise, the test has no value as a routine screen in a healthy person: a mild toxoplasmosis needs no treatment.¹

Do you need to fast?

No. The toxoplasmosis blood test simply looks for antibodies in the blood: it does not require fasting, and can be done at any time of day. You can eat and drink normally beforehand. It is often drawn at the same time as other pregnancy serologies (such as rubella). To learn about the rare tests that do require fasting, see our guide on fasting before a blood test.²

Understanding your results

The test measures two kinds of antibodies. Understanding their logic is enough to decode most of your report — bearing in mind that only your clinician makes the final interpretation.

• IgG appears a few weeks after infection and persists for life. Its presence signals older immunity, generally protective: you have already met the parasite.
• IgM appears early and may point to a recent infection. But be careful: it can stay positive for months or even years, and gives false positives. A positive IgM therefore does not prove a recent infection.⁵⁶
• The IgG avidity test is the tiebreaker. Avidity measures how tightly the antibody binds the parasite: it rises over time. High avidity early in pregnancy indicates an infection that occurred more than 3–4 months ago — so before the pregnancy, and reassuring. Low avidity does not, by itself, prove a recent infection, but calls for follow-up.⁶⁷

Note: exact cutoffs (in IU/mL) and the "positive / negative / equivocal" label vary by lab and assay. Always compare your result with the range printed on your report, and have it interpreted by your clinician, who factors in timing and any change over time.⁵¹

You are not immune (IgG−, IgM−)

You have never encountered the parasite, so you are not protected. This is not an abnormality or a cause for worry in itself — many people test this way. Outside pregnancy there is nothing to do. In pregnancy, it means prevention matters and, depending on the clinical picture, your clinician may repeat testing to catch a possible seroconversion early. It is also a good time to review your other serologies, such as rubella and CMV.⁸¹

You are immune (IgG+, IgM−)

This is the most reassuring pattern in pregnancy. Positive IgG with negative IgM signals old, stable immunity with no marker of recent infection. You are protected: the risk of passing toxoplasmosis to the baby is considered negligible. This status, once established, holds for future pregnancies too.⁴²

You have a positive IgM: don't panic

This is the result that worries people most — often needlessly. A positive IgM alone is neither proof of recent infection nor a danger to the baby. Anti-toxoplasma IgM can stay positive for a very long time after an old infection, and false positives are common. To decide, your clinician relies on:⁶⁷⁸

• the IgG avidity test: high avidity early in pregnancy rules out a recent infection;
• a second sample 2–3 weeks apart, compared with the first in the same lab (a clear rise in IgG points to recent infection);
• sometimes a reference laboratory for confirmation.

So it is not a result to interpret alone or in a rush: it is the combination of IgG + IgM + avidity + trend that gives the answer. A positive IgM does not announce disease in the baby.

What about congenital toxoplasmosis?

It occurs only if a primary infection genuinely happens during pregnancy and the parasite reaches the fetus. The chance of transmission increases as pregnancy advances, but the severity for the baby is greater when the infection is early. If a seroconversion is confirmed, specialist care is arranged (treatment, ultrasound monitoring, and sometimes PCR testing of the amniotic fluid to look for the parasite). Many exposed babies are born without any lasting problem, especially with prompt care.³⁹

Screening: why the U.S. differs from France

Approaches to toxoplasmosis in pregnancy differ by country. In France, prenatal screening is mandatory: every pregnant woman is tested early, and non-immune women are retested every month until delivery. The United States takes the opposite approach: there is no universal prenatal screening for toxoplasmosis. Because infection is less common in the U.S. and the benefit-to-cost balance is judged differently, U.S. guidance emphasizes prevention and reserves testing for specific situations — symptoms, suspicious ultrasound findings, or immunocompromise.¹⁸

Neither approach is simply "right"; they reflect different local infection rates and health-system choices. What matters for you is to follow your own clinician's advice.

If you are not immune (or unsure), a few simple habits sharply lower the risk:¹²

• cook meat thoroughly (to a safe internal temperature) and avoid raw, cured, or undercooked meat;
• wash fruits, vegetables, and herbs well, and wash your hands after handling food;
• wear gloves to garden or handle soil, and wash your hands afterward;
• have someone else change the cat's litter box if possible; otherwise wear gloves and change it daily (feces become infectious only after 1–3 days);
• you do not need to give up your cat: an indoor cat fed cooked or commercial food poses very little risk.

Recent research

According to recent PubMed-indexed publications:

• Avidity is now a well-validated tool. Studies confirm that the IgG avidity test reliably helps separate a recent from an old infection in pregnancy, with high avidity making a recent primary infection very unlikely.⁶ A 2025 systematic review and meta-analysis supports combining IgM, IgG avidity, and IgA to date the infection more accurately.⁷ (Tork M et al., BMC Pregnancy Childbirth, 2025 — DOI.)
• Prenatal diagnosis by PCR. Testing the amniotic fluid for the parasite by PCR (via amniocentesis) can confirm or rule out fetal involvement after a maternal seroconversion; real-time PCR has improved its sensitivity and reliability.⁹
• A preventable burden. Recent reviews stress that congenital toxoplasmosis remains a preventable cause of disability, and that prevention and targeted testing are the most effective levers, with most severe cases stemming from an infection that went undetected.³⁴
• An open screening debate. Whether to screen every pregnancy stays debated across countries, depending on local infection rates and how follow-up is organized — which explains why the U.S. and France differ.⁵

These findings concern diagnosis and research; they do not authorize self-treatment and do not replace your physician's advice.

Get your toxoplasmosis result interpreted by AI DiagMe

A toxoplasmosis serology is never read in isolation: its meaning depends on the IgG / IgM / avidity combination, the timing of the draw, how it changes, and your context (pregnancy, immune status). A positive IgM, in particular, only makes sense once placed within that whole picture.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

The toxoplasmosis blood test is above all a way to know whether you are immune or not immune to Toxoplasma gondii. It relies on two antibodies: IgG (older, protective immunity) and IgM (a possible recent infection, but often misleading), which the IgG avidity test helps sort out. The infection is usually mild; the real concern is a primary infection during pregnancy — which is why prevention matters and why some countries (like France) screen every pregnancy while the United States does not. Above all, remember that a positive IgM is not bad news by itself: only the combination of results and follow-up, with your clinician, gives the answer. No value is read alone — it is your whole context that counts, which is what AI DiagMe provides, alongside your physician.

Sources

Official U.S. sources and peer-reviewed publications (PubMed) used for this guide: