Infections
Legionella pneumophila antigeen
Rapid test for a fragment of the Legionella bacterium in urine; in Legionnaires' disease often positive from the third day of illness.
Everything about this testWhat it measures
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. During an infection, fragments of the bacterium, the antigen, end up in the urine. This test detects those fragments. Unlike an antibody test, it therefore does not measure the immune response but the bacterium itself, and this is possible from about the third day with symptoms. The test is mainly aimed at Legionella pneumophila serogroup 1, the type that causes most infections.
- A current Legionella pneumonia (Legionnaires' disease) caused by Legionella pneumophila serogroup 1
- A sign that a pneumonia is caused by Legionella rather than by another bacterium
Positive
A positive result means that Legionella antigen was found in your urine. The specificity of the test is almost 100 percent, so in someone with pneumonia a positive result is a strong sign of legionellosis. Symptoms that fit it are high fever, coughing, shortness of breath, muscle pain and sometimes confusion or diarrhea. According to the notification criteria, a positive result is enough to report the disease to the GGD (municipal public health service), so that the search for the source can start.
Negative
A negative result does not rule out legionellosis. The sensitivity depends on how severe the illness is and ranges from 50 to more than 90 percent; in severe pneumonia the test is usually positive, with a milder course more often negative. The test also finds almost only serogroup 1, so an infection by another Legionella species or serogroup can be missed. In the first two days with symptoms, the antigen may not yet be in the urine.
If your result is abnormal
With a positive result, the doctor starts targeted antibiotics and, if needed, has a culture done on sputum or other respiratory material, which is needed to trace the source. 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), which then investigates the possible source, for example a water system or cooling tower.
Reliability
The urine test for Legionella antigen is simple, fast and very specific: according to the LCI guideline (RIVM) on legionellosis, almost 100 percent. The sensitivity ranges from 50 to more than 90 percent, depending on the severity of the illness. The result mainly means something in people with pneumonia; in someone without respiratory symptoms, the chance of legionellosis is so small that the test adds little. A culture remains important for tracing the source, because it allows the patient and the source to be compared with each other.
Limitations
- The test is mainly aimed at Legionella pneumophila serogroup 1 and misses infections by other species or serogroups.
- The sensitivity is lower with a milder illness; a negative result does not rule out legionellosis.
- The antigen can usually only be detected from about the third day with symptoms.
- The antigen can remain in the urine for weeks after the infection, so a positive result does not always belong to the current symptoms.
How common is this
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.
Who is this useful for
Useful with pneumonia, especially with a severe course, in people who have recently been abroad or have used a whirlpool or hot tub, or with a cluster of patients. Also in people at higher risk of a severe course, such as smokers, older people and people with a lung disease, diabetes, kidney disease or reduced immunity. Not useful in people without respiratory symptoms or as a general check-up: the test is designed to explain a pneumonia, not to screen.
Downsides and risks
- A negative result can give false reassurance: with a milder course or another Legionella species, the test misses the infection.
- Because the antigen can persist for weeks, a positive result with new symptoms can wrongly be attributed to Legionella.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results