Explanation

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

Lyme disease is caused by the Borrelia bacterium, which enters the body through a tick bite. In an immunoblot, individual proteins of that bacterium are placed on a strip, so the laboratory can see which proteins the antibodies in your blood react against. IgM antibodies are the first the body makes, usually only a few weeks after infection. According to the CBO guideline on Lyme disease, the immunoblot is a confirmation test: it is meant for blood in which a screening test for Borrelia antibodies (EIA) was positive or doubtful. That screening is a separate test.

What can it detect?

Support for the diagnosis of early Lyme disease, with symptoms of short duration and after a positive or doubtful screening test

Result

What does the result mean?

Positive result

A positive IgM blot can fit a recent Borrelia infection, but does not prove active disease: antibodies show the reaction of your immune system, not whether the bacterium is active now. IgM tests give false-positive results relatively often, for example due to rheumatoid factor, a recent infection with the Epstein-Barr virus (glandular fever) or cytomegalovirus, syphilis or an autoimmune disease. IgM antibodies can also remain present for months to years after an earlier or treated infection. According to the LCI guideline (RIVM), a positive IgM without IgG is often false positive when symptoms have been present for more than a few months. The result only counts together with symptoms that fit Lyme disease.

Negative result

A negative IgM blot means that no clear IgM antibodies were found. In the first weeks after infection, the body has often not yet made antibodies; with symptoms of short duration, a negative result therefore does not rule out Lyme disease, and the guideline advises repeating the test after a few weeks. With symptoms that have been present for more than 8 weeks, IgM is less important: IgG antibodies are then almost always detectable in Lyme disease. A negative blot after a positive screening test makes Lyme disease unlikely, but does not completely rule out a very early infection.

After an abnormal result

With a positive result and matching symptoms, the GP assesses, if needed in consultation with a medical microbiologist, whether treatment with antibiotics is needed; 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 a few weeks. Lyme disease is not notifiable in the Netherlands.

Reliability

How reliable is the test?

According to the CBO guideline on Lyme disease, serological testing starts with a screening test (EIA); only a positive or doubtful result is confirmed with an immunoblot. After a negative screening test, the meaning of an immunoblot has never been systematically studied. With typical erythema migrans, the red spot or ring that grows after a tick bite, blood tests say little: the diagnosis is made by looking at it, and the antibody test is then still negative in about half of people. The presence of antibodies on its own is not proof of an active infection.

What can affect the result?

In the first weeks after infection, antibodies are not yet detectable (window period).
IgM tests sometimes give false-positive reactions due to rheumatoid factor, an acute Epstein-Barr virus or cytomegalovirus infection, syphilis or autoimmune diseases.
IgM antibodies can remain present for a long time, sometimes years, after an infection or after treatment.
Antibiotics early in the infection can prevent antibodies from developing, so that the test stays negative.

Who it is for

Who is the test useful for?

Useful for

According to the CBO guideline, meant as confirmation after a positive or doubtful screening for Borrelia antibodies (a separate test), with symptoms that fit Lyme disease: radiating nerve pain, paralysis of one side of the face, a swollen knee or heart symptoms after possible contact with ticks. Not useful with a typical red spot or ring (the GP then makes the diagnosis without a blood test), after a tick bite without symptoms, or with long-lasting vague symptoms such as tiredness without signs of Lyme disease; the chance of a false-positive result is then greater than the chance of a true one. The test is also not suitable for checking whether a treatment has worked.

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 spot or ring. About 1,500 people a year get symptoms of the joints, skin, nerves or, rarely, the heart. About 5 to 10 in 100 healthy Dutch people have antibodies against Borrelia without active disease; in people who work outdoors a lot, such as forest rangers, this is more common.

RIVM and the RIVM/LCI guideline on Lyme disease

Drawbacks and risks

A false-positive IgM result can lead to an incorrect diagnosis of Lyme disease and unnecessary antibiotic use, while another cause of the symptoms goes unnoticed.
The blot is meant as confirmation after a positive screening for Borrelia antibodies; without such screening, the meaning of a positive blot has not been well studied.

Get tested