Explanation

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

Lyme disease is caused by the Borrelia bacterium, which is transmitted through a tick bite. In an immunoblot, individual proteins of the bacterium are placed on a strip, so it becomes visible which proteins the antibodies in your blood react against. IgG antibodies develop later than IgM, usually a few weeks to months after infection, and can then remain present for years. According to the CBO guideline on Lyme disease, the immunoblot is a confirmation test after a positive or doubtful screening test for Borrelia antibodies (EIA). That screening is a separate test.

What can it detect?

Support for the diagnosis of Lyme disease that has been present for some time, such as Lyme arthritis (swollen knee), nerve symptoms or the skin condition acrodermatitis
A past Borrelia infection, even if it caused no symptoms

Result

What does the result mean?

Positive result

A positive IgG blot means that your immune system has had contact with the Borrelia bacterium at some point. This can fit Lyme disease that has been present for some time, but also an earlier infection, sometimes unnoticed or already treated, because IgG antibodies often remain present for years. About 5 to 10 in 100 healthy Dutch people have these antibodies without active disease. The result therefore only supports the diagnosis if there are symptoms that clearly fit Lyme disease; with long-lasting vague symptoms without such signs, the chance that a positive result means active Lyme disease is small.

Negative result

A negative IgG blot makes Lyme disease that has been causing symptoms for more than 8 weeks very unlikely: according to the guideline, more than 98 in 100 people with those forms have IgG antibodies. In the first weeks after infection, IgG is often still absent, so with symptoms of short duration a negative result does not rule out Lyme disease; the test can then be repeated after a few weeks. A negative blot after a positive screening test makes Lyme disease unlikely.

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, where additional tests such as a lumbar puncture or joint aspiration are sometimes done. 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, the immunoblot is meant as confirmation after a positive or doubtful screening test; after a negative screening test, the meaning of a blot has not been well studied. In Lyme disease that has been causing symptoms for more than 8 weeks, the sensitivity of IgG serology is high (more than 98 in 100), so it is mainly a negative result that is meaningful. How much a positive result says depends strongly on how likely Lyme disease already was beforehand: in people with long-lasting symptoms without typical signs, that chance is small and positive results are often not caused by active disease. Antibodies show the reaction of the immune system, not whether the bacterium is still active.

What can affect the result?

IgG antibodies often remain present for years after an infection or treatment and then say nothing about an active infection.
People who spend or work a lot in nature more often have antibodies without disease.
In the first weeks after infection, IgG is usually not yet detectable.
After treatment, the result falls just as much in people with and without lasting symptoms; the test is therefore not suitable for assessing the effect of treatment.

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, such as a swollen knee, radiating nerve pain, paralysis of one side of the face or a bluish-red, thin skin change on an arm or leg. Not useful with a typical red spot or ring after a tick bite, after a tick bite without symptoms, as a check after treatment or as a general check in people without symptoms; antibodies are then common without disease.

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

Antibodies also occur in healthy people; a positive result without matching symptoms can lead to an incorrect diagnosis of Lyme disease and unnecessary antibiotic use.
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.

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