Explanation

Toxoplasmosis IgM: what is measured?

After an infection with the parasite Toxoplasma gondii, the immune system first makes IgM antibodies and then IgG. IgM belongs to an early immune response and is therefore used as a sign of a recent infection. In toxoplasmosis, however, IgM often remains detectable for a long time, in some people even 2 years after the infection. IgM is therefore always assessed together with IgG.

What can it detect?

Possible recent toxoplasma infection (together with IgG)
A reason for further testing into when the infection happened, especially during a pregnancy

Result

What does the result mean?

Positive result

A positive IgM may fit a recent infection, but does not prove it. IgM can remain detectable for up to 2 years after an infection, and a weakly positive IgM also occurs without infection (false positive). If IgG is negative and IgM positive, this may be a very fresh infection or a false positive result; a second blood sample after a few weeks makes this clear. If IgG and IgM are both positive, an avidity test can show whether the infection happened more than 4 months ago. A recent infection usually causes no or mild symptoms, such as swollen lymph nodes in the neck, fever, muscle pain or tiredness.

Negative result

A negative IgM makes a recent infection unlikely. Together with a positive IgG, this fits an infection from longer ago; together with a negative IgG, it usually means you have never been infected. Very shortly after an infection, IgM can still be negative.

After an abnormal result

With a positive IgM, the GP or midwife usually has blood taken again after a few weeks or has the IgG avidity measured. If there are signs of a recent infection during pregnancy, referral to the gynecologist follows, who will discuss amniotic fluid testing if needed. Toxoplasmosis is not notifiable.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on toxoplasmosis, an IgM test is not suitable for establishing a recent infection, because IgM remains detectable for a long time, and it is also not suitable for screening during pregnancy. A positive IgM is a reason for further testing, not a diagnosis. The time of infection is established with a second blood sample (a rise in antibodies) or an avidity test.

What can affect the result?

IgM can remain detectable for months up to 2 years after the infection.
Weakly positive or borderline results occur without a recent infection; tests differ between laboratories.
Pregnancy and treatment with spiramycin slow down the maturing of avidity, which makes follow-up tests harder to interpret.

Who it is for

Who is the test useful for?

Useful for

Useful with symptoms that may point to a recent toxoplasmosis, such as swollen lymph nodes in the neck with tiredness or fever, for a pregnant woman with a suspected recent infection, and for people with a severely weakened immune system. Not suitable as a stand-alone screening test when planning a pregnancy or during a pregnancy without symptoms: according to the LCI guideline, IgG is then sufficient, because a positive IgM often belongs to an old infection.

How common is it?

There are no good Dutch figures on how often a first toxoplasma infection occurs during pregnancy. About 18 in 100 women of childbearing age have already had the infection; the others can still become infected. According to an estimate from 2006, about 2 in 1000 children are born with a congenital toxoplasma infection.

The LCI guideline (RIVM) on toxoplasmosis

Drawbacks and risks

A positive IgM without a recent infection is common and can cause a lot of worry and extra testing, especially during a pregnancy.
A negative IgM very shortly after an infection can give false reassurance.

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