At a Glance

Why Get Tested?

To detect and diagnose Legionella bacteria as the cause of pneumonia or flu-like symptoms; to investigate an outbreak caused by Legionella

When To Get Tested?

When you have a cough, body aches, shortness of breath, headache, and fever with chills, and may have abdominal pain, diarrhea, nausea, and/or mental changes that a healthcare provider suspects may be due to an infection caused by Legionella

Sample Required?

A urine sample, a sputum sample, blood sample, and/or respiratory secretions or other body fluids collected by a healthcare practitioner

Test Preparation Needed?


What is being tested?

Legionella is a type of bacteria that can cause a serious form of pneumonia called Legionnaires disease. Legionella testing detects the presence of the bacteria or parts of it (antigens) to help diagnose the cause of a person’s pneumonia.

A legionella infection affects the lower respiratory tract, often requires hospitalization, and has a significant mortality rate. In addition to pneumonia, Legionella bacteria can also cause Pontiac fever, a milder and self-limiting illness with flu-like symptoms (without pneumonia). Together, Legionnaires disease and Pontiac fever are referred to as legionellosis.

Legionella bacteria are found naturally throughout the environment. They prefer warm, stagnant water and can grow in the plumbing systems of large buildings such as hotels, hospitals, and cruise ships. The bacteria may contaminate whirlpool spas, drinking and bathing water, hot water tanks, air conditioning cooling towers, ice machines, humidifiers, and public fountains.

Infections occur when an individual inhales airborne water droplets such as steam or mist that are contaminated with Legionella bacteria. The bacteria are not passed from one person to another, but many people can become infected by the same contaminated water source, potentially resulting in a legionellosis outbreak. According to the Centers for Disease Control and Prevention (CDC), legionellosis outbreaks occur any time two or more people become ill in the same place at about the same time.

Legionella infection or Pontiac fever is confirmed when a person showing typical symptoms of the disease has a positive confirmatory laboratory test (usually a positive culture or urine test).

State and local health departments have jurisdiction over legionellosis outbreaks in their state; however, legionellosis is a nationally notifiable disease and state health departments should report all laboratory-confirmed cases to the CDC.

Pontiac fever typically develops within 24-48 hours while Legionnaires disease develops within a few days to a couple of weeks. According to the CDC, about 8,000 to 18,000 people are hospitalized with Legionnaires disease each year. The actual number of people affected by legionellosis is likely much higher as most cases of Pontiac fever and many cases of Legionnaires disease are thought to go undiagnosed.

Although anyone may be affected, adults over 50 are at the highest risk for Legionnaires disease, as are those who smoke, have lung diseases, or have compromised immune systems (such as those with HIV/AIDS, cancer, or who are taking immunosuppressant medications). People with chronic underlying diseases such as diabetes, kidney disease, or liver disease are also more vulnerable, and men are affected more often than women. Most cases of Legionnaires disease are sporadic, but they may also be seen in outbreaks, which typically occur during the summer and early fall. Pontiac fever is frequently linked with outbreaks.

There are many different types of Legionella bacteria, but only a few are common. In the United States, as many as 80% of legionella infections in adults are caused by Legionella pneumophila, serogroup 1 (a subtype of the species). Other serogroups of L. pneumophila, including 4 and 6, cause a number of cases. Other species of Legionella, such as Legionella micdadei, Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae, may cause infections in children and/or are more prevalent in other parts of the world.

How is the sample collected for testing?

The sample collected depends on the test to be performed.

  • For antigen testing, a random urine sample is collected in a sterile cup provided by the laboratory or healthcare practitioner’s office.
  • For culture or molecular testing, a sputum sample may be expectorated or induced. Expectorated samples are coughed up and expelled into a sterile cup provided by the laboratory or health practitioner’s office. The person’s mouth should be rinsed with water or saline prior to sample collection. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Respiratory secretions, other body fluid samples, and any tissue biopsy samples are collected by a health care practitioner.

Common Questions

How is it used?

Testing is performed to detect the presence of Legionella bacteria to identify the cause of a person’s symptoms. Tests are primarily used to diagnose Legionnaires disease and distinguish it from other causes of pneumonia and/or other conditions that have similar symptoms. Since Legionnaires disease can often require hospitalization and has a significant mortality rate, it is important for a healthcare practitioner to identify it and initiate appropriate treatment with antibiotics.

Typically, one or more of the following tests may be ordered. According to the Centers for Disease Control and Prevention (CDC), a urine test for the Legionella antigen and Legionella culture of a respiratory sample are the preferred tests to diagnose Legionnaires disease and are used to confirm cases showing typical clinical features of Legionellosis.

  • Legionella pneumophila antigen—this is the preferred initial test for Legionnaires disease. It detects one of the bacterium’s proteins. It is performed on urine and occasionally on another body fluid. It is a rapid way to detect an infection, but it will only detect Legionella pneumophila serogroup 1. Since this is the most common cause of Legionnaires disease in the U.S., it will detect most infections in adults.
  • Legionella culture—this type of culture may be performed along with a routine sputum culture because special nutrient media is required to encourage the growth of Legionella and discourage the growth of other bacteria. This test is considered the “gold standard” for diagnosing an infection caused by Legionella bacteria. A positive culture may be determined in about 48 to 72 hours. Negative cultures are held for at least 7 days before a final result is reported. A culture will identify multiple Legionella species and is used both to confirm that a person has a legionella infection and to help identify the source of an outbreak.
  • Direct fluorescent antibody (DFA) staining for Legionella species—this is a rapid test that can be performed on respiratory samples and tissue and requires only 2 to 4 hours for results.
  • Legionella species by polymerase chain reaction (PCR)—this test detects bacterial genetic material, primarily in respiratory secretions but sometimes in other body fluids. It can detect several species of Legionella but is not widely used.

When is it ordered?

Legionella testing may be ordered when a person has pneumonia or symptoms associated with pneumonia, especially when the person also has gastrointestinal symptoms such as diarrhea and/or nausea and mental changes such as confusion.

The Centers for Disease Control and Prevention (CDC) recommends testing when a person:

  • Has severe pneumonia, especially if the person is in intensive care
  • Has pneumonia and has a weakened immune system
  • Has been treated with antibiotics in an outpatient setting but is still ill
  • Has pneumonia during a legionellosis outbreak
  • Has become ill and traveled away from home within the past two weeks
  • Is suspected of acquiring pneumonia in a healthcare setting, such as a hospital or nursing home

Signs and symptoms of Legionnaires disease typically include:

  • Cough (dry or productive, may have bloody sputum)
  • Headache
  • High fever, chills
  • Weakness, fatigue
  • Muscle aches
  • Chest pain
  • Shortness of breath
  • Lethargy
  • Mental changes, confusion

and may include:

  • Watery diarrhea
  • Abdominal pain
  • Nausea and vomiting

and sometimes:

  • Low blood pressure
  • Changes in heart rate

Additional symptoms may be experienced if Legionella bacteria also infect other organs such as the heart, brain, liver, or kidney.

What does the test result mean?

Results of testing must be carefully interpreted and evaluated in conjunction with a complete medical history, physical exam, and other tests such as a chest X-ray for pneumonia.

Negative legionella test results, of any type, do not rule out a legionella infection. A person may be infected by a Legionella species other than the one(s) tested or the bacteria may be present in the sample in numbers too low to be detected or may not have grown in culture. Repeat testing may be required.

If a Legionella pneumophila urine antigen test is positive, then it is likely that a person has a Legionella pneumophila serogroup 1 infection. If the person has symptoms consistent with pneumonia, then it is likely that the person has Legionnaires disease.

If a legionella culture is positive, then a person is confirmed as having a legionella infection and whatever species is present in the culture is identified as the cause.

If a Legionella species by PCR genetic test is positive, then it is likely that the person has a legionella infection. However, there are a small number of false positives associated with this test.

Is there a blood test for Legionella?

Blood tests for Legionella pneumophila antibody are no longer routinely performed but may be available from some reference laboratories. The test detects antibodies, the body’s immune response to a legionella infection, in the blood. It is not useful for immediate clinical decision-making but can be ordered to confirm a recent infection. Usually two blood samples taken weeks apart are tested, and an increase in the level of antibodies against the bacteria indicates an active infection. A single positive antibody test cannot be used to diagnose a recent infection as up to 15% of the general population may have legionella antibodies in their blood, indicating exposure to the bacteria at some point in their life. Some people, such as those who have compromised immune systems, can have a legionella infection without having a normal antibody response.

Is there anything else I should know?

Legionella pneumophila by direct fluorescent antibody (DFA) is a test that is no longer routinely recommended. It is not as sensitive as other testing, is labor-intensive, and requires a relatively large respiratory sample.

Besides tests for Legionella pneumophila, what other testing may be done?

Other tests may be performed to help evaluate a person’s overall health status and to distinguish between conditions that cause similar symptoms. These may include:

Legionnaires disease can affect other blood test results. These tests are not used to help diagnose a legionella infection but may be additional clues for the healthcare provider and information about the person’s health status. Often seen are:

And also:

  • Blood and protein in the urine
  • On a Gram stain slide under the microscope, the presence of white blood cells but few or no bacteria. If Legionella bacteria are present, they typically appear as small, pale, Gram-negative (pink stained) variable-shaped rods.

Can water be tested for Legionella bacteria?

Yes, and it frequently is tested to monitor complex water systems, to detect the source of legionellosis outbreaks, and to monitor the effectiveness of water treatment when Legionella bacteria are detected.

Can Legionella bacterial contamination and infection be prevented?

The opportunities for them to occur can be minimized, but the risk cannot be totally eliminated. Legionella are very common bacteria in the environment. In building water systems, they can resist low levels of chlorine treatment and can persist in sediment found in pipes. Home and public hot tubs should be properly disinfected and pH of the water regularly checked and maintained at appropriate levels. Individuals at increased risk for infection may choose to avoid high-risk exposures, such as being in or near a hot tub.

In Legionella outbreak situations, hyperchlorination of hospital water (using high levels of chlorine) or raising the temperature of the hot water to over 70⁰C, or a combination of both methods has been proven effective for suppressing the growth of Legionella.

Should everyone with flu-like symptoms be tested for Legionella?

Pontiac fever, the type of legionella infection that causes flu-like systems, is generally self-limiting and resolves in most cases without treatment, so testing is not considered routinely necessary. Testing might be warranted in cases where Legionnaires disease symptoms are associated with travel, hospitalization, a stay in an assisted-living facility, or other risk factors (e.g., smoking, older than age 50, chronic lung disease, etc).

Why is it called Legionnaires disease?

The disease and bacteria were named after an incident in which a significant number of people who went to an American Legion convention in Philadelphia in 1976 developed pneumonia. A new type of bacteria was identified as the cause and was subsequently named Legionella. This outbreak caused illness in 182 people and resulted in 29 deaths.


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