Explanation

Legionella species IgM: what is measured?

Legionella is a bacterium that lives in water and that people take in by breathing in tiny water droplets, for example from a shower, a whirlpool or a cooling tower. This test measures IgM antibodies: the type of antibody the immune system makes first. With Legionella, this distinction is less useful than with many other infections, because according to the LCI guideline (RIVM), IgM titers can persist for a relatively long time and so do not always point to a recent infection.

What can it detect?

An immune response to a Legionella infection, without it being clear how recent it was
Support for a diagnosis afterwards, together with IgG and a second blood sample

Result

What does the result mean?

Positive result

A positive IgM result means that antibodies against Legionella bacteria were found. Unlike with many other infections, this does not reliably say that the infection was recent, because IgM against Legionella can persist for a long time. The result only counts if there are symptoms that fit Legionella pneumonia, such as high fever, coughing, shortness of breath, muscle pain and sometimes confusion or diarrhea, and it must be confirmed with other tests.

Negative result

A negative IgM result does not rule out a Legionella infection. The sensitivity of Legionella serology is about 70 to 80 percent, and antibodies only appear weeks after the start of the illness. When an acute pneumonia is suspected, a urine test for Legionella antigen or a PCR on material from the lower airways is much more useful.

After an abnormal result

When a past infection is suspected, the doctor compares the result with a second blood sample; with a current pneumonia, testing of urine or respiratory material follows and, if needed, treatment with antibiotics. Legionellosis is a notifiable group C disease under the Dutch Public Health Act (Wet publieke gezondheid) and must be reported to the GGD (municipal public health service) within one working day.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on legionellosis, serology is not useful for acute diagnosis, nor for monitoring treatment; the test is mainly valuable for epidemiological research and for a diagnosis afterwards. The sensitivity is around 70 to 80 percent and cross-reactions with Mycoplasma and Chlamydia are known. Because IgM titers can persist for a long time, this test distinguishes less well between a recent and an older infection than with other infections.

What can affect the result?

IgM titers against Legionella can persist for a long time and therefore do not always point to a recent infection.
Antibodies only appear weeks after the start of the illness; the test is not suitable for an acute pneumonia.
The sensitivity is about 70 to 80 percent, so a negative result does not rule out the infection.
Cross-reactions with antibodies against Mycoplasma and Chlamydia are known, especially with the micro-agglutination test.

Who it is for

Who is the test useful for?

Useful for

Can be useful as an addition to IgG to support a past Legionella infection afterwards, for example when tracing the source or with a pneumonia whose cause was not found. Not useful with an acute pneumonia: for that, the urine test for Legionella antigen, a culture or a PCR are the appropriate tests. Also not useful as a general check-up in people without symptoms.

How common is it?

In the Netherlands, 450 to 650 cases of Legionella pneumonia are reported to the GGD (municipal public health service) each year, about 2.6 to 3.8 per 100,000 inhabitants. About a third of these people were traveling during the incubation period and probably caught the infection abroad. It is estimated that 1 to 7 in 100 cases of pneumonia for which someone is admitted to hospital are caused by Legionella. Most reports are in summer.

RIVM/LCI guideline on legionellosis

Drawbacks and risks

Because IgM persists for a long time, a positive result can wrongly be read as a recent infection and lead to unnecessary antibiotic use or unnecessary source tracing.
A negative result can give false reassurance to someone who does have Legionella pneumonia.

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