Explanation

Babesia microti IgG: what is measured?

Babesia is a single-celled parasite related to the malaria parasite that invades red blood cells. People can pick up the parasite through a tick bite, and very rarely through a blood transfusion. This test measures IgG antibodies against Babesia microti, the species that mainly causes disease in humans in North America. IgG appears later than IgM and then remains detectable for a long time, so the test mainly shows contact with the parasite, not whether it is still present now.

What can it detect?

Past contact with the parasite Babesia microti
Support in investigating babesiosis with fever and anemia due to breakdown of red blood cells after a tick bite or a stay in North America

Result

What does the result mean?

Positive result

A positive result means that antibodies against Babesia microti were found. This says that your immune system has had contact with the parasite at some point, not that you are infected now; antibodies remain detectable for a long time. In the Netherlands, babesiosis in humans hardly ever occurs, so in someone who has never been in an area where the disease is more common, a positive result is more often wrong than right. To show an active infection, the laboratory looks for the parasite itself in a blood smear or with a PCR.

Negative result

A negative result makes a past infection with this parasite unlikely. In the first week after infection, the test can still be negative even though there is an infection; IgG only appears after a few weeks. With acute symptoms of fever and anemia, a blood smear or PCR is the appropriate test, not an antibody test.

After an abnormal result

With a positive result and matching symptoms, the doctor has a blood smear or PCR done to see whether the parasite is really present, usually in consultation with a medical microbiologist; with a severe course, treatment in hospital follows. Babesiosis is not notifiable in the Netherlands.

Reliability

How reliable is the test?

There are no agreed limit values for these antibodies, and the meaning of a positive result in people in the Netherlands has been little studied. Babesiosis in humans hardly ever occurs in the Netherlands: the first patient who caught the infection in the Netherlands was reported in September 2023. In addition, this test targets Babesia microti, while the Babesia species in Dutch ticks are usually other species, so an infection here can also be missed. An antibody test shows the immune response, not the parasite; an active infection requires microscopic examination or PCR.

What can affect the result?

IgG antibodies remain detectable for a long time after contact and say nothing about an active infection.
The test targets Babesia microti; other Babesia species found in Dutch ticks may be missed.
In the first weeks after infection, the result can still be negative.
Cross-reaction with antibodies against related parasites, including malaria parasites, is possible.

Who it is for

Who is the test useful for?

Useful for

Can be useful with fever and anemia due to breakdown of red blood cells, jaundice or dark urine, in someone who has had a tick bite or has been in North America, and especially in people without a spleen or with a weakened immune system. Not useful as a general check after a tick bite or in people without symptoms: the disease hardly ever occurs in the Netherlands and a positive result is then almost always wrong.

How common is it?

Babesiosis in humans hardly ever occurs in the Netherlands. A small proportion of Dutch ticks carry the parasite, but the first patient who caught the infection in the Netherlands was only reported in September 2023. Many infected people get no symptoms; if they do, these are usually flu-like symptoms with chills, fever, sweating, nausea and tiredness. People without a spleen, with a weak immune system or with another illness have a higher chance of a severe course.

RIVM

Drawbacks and risks

Because the disease hardly ever occurs in the Netherlands, a positive result in someone without symptoms is almost always wrong and leads to unnecessary worry and unnecessary further testing.
A negative result can give false reassurance, because the test targets one species and can miss other Babesia species.

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