Explanation

Toxoplasmosis IgG: what is measured?

Toxoplasmosis is an infection with the parasite Toxoplasma gondii, which you can catch from raw or undercooked meat, cat feces or soil. After infection, the immune system makes IgG antibodies (immune proteins). These remain in the blood for life, just as the parasite itself remains dormant in the body. A positive IgG therefore shows that you have had the infection at some point, but not when.

What can it detect?

Past toxoplasma infection, with immunity against a new infection
No previous infection: susceptible to a first infection, important when planning a pregnancy or during pregnancy
Together with IgM and an avidity test: an indication of the time of infection

Result

What does the result mean?

Positive result

A positive result means that you have been infected with Toxoplasma at some point. Usually this happened long ago and went unnoticed. If you already have IgG before pregnancy, according to the LCI guideline (RIVM) you are protected: there is then generally no risk that the unborn child becomes infected. Only if IgM is also present, or if no IgG was found in an earlier test, can it be a recent infection; additional tests are then needed to determine the time of infection.

Negative result

A negative result usually means that you have never had toxoplasmosis and therefore have no immunity to it. This is very common: well over 80 in 100 women of childbearing age have no antibodies. If you are not pregnant and have a healthy immune system, this has no consequences. During pregnancy, however, a first infection can be harmful to the child; the advice to prevent infection (cooking meat thoroughly, gloves when gardening and when cleaning the litter box) is then extra important. Shortly after an infection the test can still be negative, because antibodies only become detectable after some time.

After an abnormal result

A positive IgG before pregnancy needs no follow-up. If the test was done during pregnancy, the midwife or GP usually also has IgM and, if needed, the avidity measured, or compares the result with blood taken earlier; if there are signs of a recent infection, referral to the gynecologist follows. Toxoplasmosis is not notifiable.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on toxoplasmosis, measuring IgG is sufficient for the question whether someone has ever had toxoplasmosis. The test reliably shows whether someone has been infected, but not when. To estimate the time of infection, the laboratory uses IgM, a second blood sample a few weeks later (antibodies newly appearing or rising) or the avidity of IgG: a high avidity means that the infection was at least 4 months ago.

What can affect the result?

In the first weeks after infection, IgG antibodies are not yet detectable.
Laboratories use different tests and cut-off values; a weakly positive result is sometimes hard to interpret.
With a severely weakened immune system, for example after a transplant, the antibody test says less.
After a blood transfusion or administration of immunoglobulins, antibodies from donors can temporarily be included in the measurement.

Who it is for

Who is the test useful for?

Useful for

The result means the most for women planning a pregnancy or who are pregnant and want to know whether they are already protected, for example when working with cats, raw meat or soil; under the Dutch Working Conditions Decree (Arbobesluit), pregnant employees must not be exposed to the parasite unless they are immune. The result is also important for people with a severely weakened immune system. Without such a reason, the result has fewer consequences, because the advice to prevent infection during pregnancy applies to every pregnant woman who does not know for sure whether she is immune. After a single contact with a cat or raw meat, a test says little; explanation and advice to prevent infection are more helpful then.

How common is it?

In the Netherlands, about 30 in 100 people had antibodies against Toxoplasma in 2016 and 2017. This rises from about 10 in 100 under the age of 20 to more than 80 in 100 in the oldest age group; among women of childbearing age it was about 18 in 100. According to an estimate from 2006, about 2 in 1000 children are born with a congenital toxoplasma infection, usually without this being noticed.

LCI guideline on toxoplasmosis (RIVM), based on the Pienter study

Drawbacks and risks

A positive result during pregnancy can cause worry and extra blood draws, while it is usually an old, harmless infection.
A negative result does not protect you: if you have no antibodies, you can still become infected after the test.

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