Explanation

Borrelia IgM antibodies (Lyme disease): what is measured?

Lyme disease is caused by the Borrelia bacterium, which is transmitted through a tick bite. This test measures IgM antibodies: the type of antibody the immune system makes first, usually only three to six weeks after the bite, and which then decreases again. IgG comes later and lasts longer. According to the CBO guideline on Lyme disease, testing takes place in two steps: a positive or doubtful screening test is confirmed with an immunoblot on the same blood sample.

What can it detect?

Antibodies that may fit a recent Borrelia infection, for example with early nerve symptoms or several red spots at the same time
An immune response to a tick bite that happened a few weeks ago

Result

What does the result mean?

Positive result

A positive IgM result can fit an infection that is a few weeks old, but on its own says little. According to the LCI guideline (RIVM), a single positive IgM result with symptoms that have been present for more than a few months is often false positive. The result must be confirmed with an immunoblot and assessed together with the symptoms. IgM can also appear with other infections, such as the glandular fever virus, and with autoimmune diseases, without there being Lyme disease.

Negative result

A negative IgM result means that no IgM antibodies were found. Shortly after a tick bite, this says little: the antibodies only become detectable three to six weeks after the bite. With symptoms of short duration, the test can therefore be repeated after 4 to 8 weeks. With symptoms that have been present for more than 8 weeks, IgG is the test that counts; IgM has then often already disappeared and a negative IgM result does not rule out Lyme disease.

After an abnormal result

With a positive or doubtful result, the GP has an immunoblot done on the same sample and the result is assessed together with the symptoms and the IgG result; with symptoms of the nerves, joints or heart, referral to hospital usually follows. With symptoms of short duration and a negative result, the test can be repeated after 4 to 8 weeks. Lyme disease is not notifiable in the Netherlands.

Reliability

How reliable is the test?

IgM is the less reliable of the two Borrelia antibodies. The CBO guideline on Lyme disease prescribes two-step testing: first a screening test, then an immunoblot for confirmation. With a longer duration of illness, a single positive IgM is often a false-positive result, according to the LCI guideline (RIVM), while in an early infection with a red ring or spot, the sensitivity of serology is only about 50 percent. A positive result only has meaning if there are symptoms that fit recent Lyme disease.

What can affect the result?

In the first three to six weeks after the tick bite, there are often no antibodies yet (window period).
IgM can also rise with other infections (for example the glandular fever virus or cytomegalovirus) and with autoimmune diseases; cross-reactions are a known problem.
IgM can remain detectable for a long time after a past infection and then wrongly seem to point to something recent.
Dutch laboratories use different tests; with symptoms of short duration, the results do not always agree.

Who it is for

Who is the test useful for?

Useful for

Useful when recent Lyme disease that has spread beyond the skin is suspected, for example with several red spots at the same time, radiating nerve pain or paralysis of one side of the face, and then always together with IgG. Not useful with a typical red ring or spot after a tick bite, with a tick bite without symptoms, or with long-lasting vague symptoms: a positive IgM is then usually false positive. As a check after treatment, the result says little, because antibodies can remain detectable for a long time after treatment.

How common is it?

In the Netherlands, people together get about 1.5 million tick bites a year and about 27,000 people a year get Lyme disease, usually only with a red ring or spot. About 1,500 people a year get symptoms of the joints, skin, nerves or, very occasionally, the heart. About 5 to 10 in 100 healthy Dutch people have antibodies against Borrelia without active disease.

RIVM and the RIVM/LCI guideline on Lyme disease

Drawbacks and risks

A single positive IgM result is often false positive with longer-standing symptoms and can lead to an incorrect diagnosis of Lyme disease and unnecessary antibiotic use.
Because many people have had a tick bite, a positive result quickly feels like an explanation for symptoms that in reality have another cause.

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