Explanation

Toxoplasma gondii antibodies: what is measured?

This screening test shows whether there are antibodies (immune proteins) against the parasite Toxoplasma gondii in the blood. After an infection, the body first makes IgM and then IgG; IgG remains present for life, just as the parasite remains dormant in the body. The screening test does not distinguish between an old and a recent infection. With a positive result, IgG and IgM can be measured separately.

What can it detect?

Past or recent toxoplasma infection
No antibodies: susceptible to a first infection, important when planning a pregnancy or during pregnancy

Result

What does the result mean?

Positive result

A positive screening test means that antibodies against Toxoplasma have been found. Usually this is an infection from long ago that went unnoticed; if you have this before pregnancy, according to the LCI guideline (RIVM) you are generally protected and the unborn child is then not at risk. A recent infection cannot be ruled out with this test. Additional tests are needed for that (IgG, IgM and possibly avidity), especially during pregnancy or with symptoms such as swollen lymph nodes in the neck.

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 screening test before pregnancy usually needs no follow-up. During pregnancy or with symptoms, the GP or midwife has IgG and IgM measured separately and, if needed, the avidity; if there are signs of a recent infection during pregnancy, referral to the gynecologist follows. Toxoplasmosis is not notifiable.

Reliability

How reliable is the test?

The screening test answers the question whether someone has ever been infected with Toxoplasma; according to the LCI guideline (RIVM) on toxoplasmosis, an antibody test is suitable for this. The time of infection cannot be derived from it. Exactly which antibodies the screening test measures and which cut-off value is used differs between laboratories.

What can affect the result?

In the first weeks after infection, antibodies may not yet be detectable.
A weakly positive result is sometimes hard to interpret and calls for additional tests.
With a severely weakened immune system, for example after a transplant, the antibody test says less.

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 screening test during pregnancy can cause worry and extra blood draws, while it is usually an old 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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