Toxoplasmosis avidity test in pregnancy: IgM and IgG antibodies

Let’s deepen the tests for toxoplasmosis in pregnancy with the dosage of igM and igG antibodies (test of avidity) to understand whether or not the mother is immune or if the infection is in progress.

Toxoplasmosis is caused by a parasite, Toxoplasma Gondii, which completes its life cycle in the host. The parasite can infect from molluscs to mammals (including humans) through ingestion of infected raw meat , direct or indirect contamination with infected cat feces through soil.

In fact, during pregnancy the infection can pass through the placenta and in some cases it can cause malformations or even miscarriage or stillbirth.

For this reason,  women seeking a child are advised to check their immunity to the parasite even before pregnancy , so that they can already take the necessary precautions even before finding themselves pregnant.

The exam is called Toxo-Test , it takes place with a normal blood sample

Once pregnant, if the woman is not immune, the checks are repeated frequently and free of charge every 4-6 weeks, in order to be able to intervene promptly in the event of infection.

Toxoplasmosis usually has no obvious symptoms and the only way to tell if you have been infected is through a blood test.

Toxoplasmosis infection before pregnancy

If an acute Toxoplasmosis infection is found during pre-conception check-ups , it is recommended to wait 6 months before trying to conceive.

The diagnosis of toxoplasmosis in pregnancy

The Toxo-Test detects the presence of IgG and IgM in the blood.

The presence of a positive IgG value indicates that there has been an infection. If the IgM is simultaneously negative, a recent infection is excluded.

However, if the IgM is positive or has a doubtful value, other in-depth tests are needed to understand the moment of infection. It is not always possible to trace the period of acute infection as IgM can be detected in the blood up to 18 months after the infection.

It is advisable to undergo a Toxo Test as soon as pregnancy is discovered and then to repeat the tests during pregnancy always in the same laboratory.

IgM: are produced a little less than 2 weeks after infection, reach their peak 2-4 weeks after infection and then decrease, even if they are very often found in the circulation even after 9-10 months after infection. Sometimes we still find them more than a year after the infection.

IgG:  They are found in the circulation about 2 weeks after infection, reach their peak 2-3 months later and then decline, even if they can remain at low levels for life. They can increase in case of new contact with the parasite or for a reactivation (in immunosuppressed patients).

Possible results of the Toxotest:

Negative IgG and negative IgM : there is no infection in progress and you are not immune to Toxoplasmosis. It is necessary to adopt the precautions listed above and repeat the test periodically.

IgG positive and IgM negative : The patient has had the infection before. Therefore you no longer have to repeat checks unless there are situations of immunosuppression in which reactivations can occur.

IgG negative and IgM positive: 

This situation can be explained in two ways

1) False positive for IgM

2) Possible acute infection in progress (there has not yet been seroconversion, i.e. the IgG has not yet been produced).

It would be advisable to carry out a second level test as well as repeat the tests after a few days to understand which of the two hypotheses is the valid one. In the meantime, the start of therapy with the specialist is being evaluated.

IgG positive and IgM Positive: in these cases, given that IgM, as we have said, can persist in the circulation for several months after the acute infection, a second level test is prescribed. In this way it is possible to understand if the infection was prior to the pregnancy or, if pregnant, evaluate the fetal risk and how to manage the pregnancy.

In fact, in the event of infection during pregnancy, precise protocols are implemented which include prenatal diagnosis, therapy, and then the control and follow-up of the newborn once born.

Dating the infection with second-level and avidity tests

In the event that positive IgG and IgM are detected by the Toxo Test, one usually resorts to the dosage of IgA (which is however not very specific), to immunoblots and to the IgG avidity test

The presence of IgA indicates a recent infection less than six months old.

Immunoblot tests can detect early-stage seroconversions

From the igG avidity test, on the other hand, we can obtain three results:

Low/Weak Avidity (0-20%): probable ongoing infection

Intermediate avidity (20-30%): probable recent infection

High/Strong avidity (>30%):  infection most likely more than 4 months ago

Both intermediate and low avidity require further investigations and the implementation of a therapy.

The risk of transmission to the fetus increases as the pregnancy progresses; on average 17% in the first quarter, 25% in the second and 65% in the third. The later the pregnancy is, the less important are the clinical consequences for the fetus once the infection is transmitted.

The risk of congenital disease is 13%, of which 7% is severe and 6% is mild.


Currently available antibiotic therapies (eg spiramycin) have not yet demonstrated 100% efficacy in preventing transmission to the fetus. Efficacy is highest if therapy is started as soon as possible after maternal infection.

In case of confirmed fetal infection, the combination of pyrimethamine-sulfadiazine (+ folate) is currently used (but not in the first trimester of pregnancy).

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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