Testing is performed to detect the presence of Legionella bacteria so as 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 health practitioner to identify it and initiate appropriate treatment with antibiotics. (See the "What is being tested?" section for more on Legionnaires disease and outbreaks.) Photo: Legionella pneumophilia, Source: CDC/ Janice Haney Carr

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.

• 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.

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:

• Sputum culture
• Gram stain
• AFB testing
• Complete blood count
• Chemistry panel

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.

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.

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:

• Decreased sodium
• Increased phosphate
• Increased ESR and CRP
• Increased liver enzymes

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.

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. 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.

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.