Testing is performed to detect the presence of Legionellabacteria so as to identify the cause of a person's symptoms. Tests are primarily used to diagnose Legionnaires disease and to 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 doctor to identify it and initiate appropriate treatment with antibiotics.
Typically one or more of the following tests are ordered:
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.
Legionella pneumophilaantigen—this test 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.
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:
Legionella testing is 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.
Symptoms of Legionnaires disease typically include:
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 doctor and information about the person's health status. Often seen are:
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 appears as small, pale, Gram-negative (pink stained) variable-shaped rods.
Tests for Legionella pneumophilaantibody 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 decisionmaking 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.
This article was last reviewed on June 23, 2012. | This article was last modified on October 8, 2014.
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