Explanation

Legionella species IgG: 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. The bacterium can cause a severe pneumonia, Legionnaires' disease. This test measures IgG antibodies against Legionella bacteria. IgG appears later than IgM and then remains detectable for a long time, so it mainly shows that your immune system has had contact with the bacterium at some point.

What can it detect?

An immune response to a past Legionella infection
Support for a diagnosis afterwards, if the antibodies clearly rise between two blood samples

Result

What does the result mean?

Positive result

A positive IgG result means that your immune system has had contact with Legionella bacteria. That can go with a past pneumonia, but also with an infection you did not notice. According to the LCI guideline (RIVM), a single high titer can make the diagnosis more likely in someone with a matching medical history, but that is not proof; since the revision, a single high antibody level no longer counts as a criterion for notification. Only a clear rise between two blood samples is conclusive.

Negative result

A negative IgG 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 than an antibody test.

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): the doctor or laboratory must report it within one working day to the GGD (municipal public health service), after which the source is traced.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on legionellosis, serology is mainly valuable for epidemiological research and for making a diagnosis afterwards; the test is not useful for acute diagnosis, nor for monitoring treatment. The sensitivity is around 70 to 80 percent. With the micro-agglutination test, cross-reactions with Mycoplasma and Chlamydia are known. In people without symptoms, a positive result mainly says something about the past and does not change the management.

What can affect the result?

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.
Serology is not suitable for checking whether a treatment works.

Who it is for

Who is the test useful for?

Useful for

Can be useful to confirm a past Legionella infection afterwards, for example when tracing the source after an outbreak or with a pneumonia whose cause was not found, and then with two blood samples. 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

A positive result without matching symptoms can lead to unnecessary further testing and to source tracing that turns out not to have been needed.
A negative result can give false reassurance to someone who does have Legionella pneumonia, because the test misses some of the infections.

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