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Legionella Tests

Legionella pneumophila is a waterborne bacterium responsible for Legionnaires’ disease, which is an important cause of both community-acquired and nosocomial pneumonia. Legionnaires’ disease exhibits a seasonal pattern, with peak activity in warm, rainy, and humid conditions. It affects primarily older patients, especially those who are cigarette smokers, immunocompromised, or have chronic lung disease. Mortality rate is approximately 9% in community acquired cases, but can be as high as 25 to 46% in hospital acquired cases. Legionnaires’ disease cases are reportable to CDC.

Legionella grows in building water systems that are not adequately managed, especially those where disinfectant levels are low, water is stagnant, or water temperatures are optimal for growth. Legionnaires’ disease occurs most commonly after inhalation of Legionella-containing aerosols from showerheads, medical equipment, cooling towers, hot tubs, hydrotherapy equipment, or decorative fountains. Less commonly, disease occurs from aspiration of Legionella-containing water. Only one case of probable person-to-person transmission has been reported.

Nonspecific laboratory abnormalities include thrombocytopenia, leukocytosis, proteinuria, hematuria, elevated ferritin and hypophosphatemia. Hyponatremia occurs more frequently in Legionnaires' disease than in pneumonias of other etiologies. Plasma procalcitonin is usually elevated.

Legionella pneumophilia is an aerobic gram negative bacillus. Specific tests for legionella infection include respiratory culture, direct fluorescent antibody (DFA) staining of respiratory specimens, serum antibody, urine antigen and PCR on respiratory specimens. The gold standard for diagnosis of legionellosis is culture. However, growth of Legionella requires buffered charcoal yeast extract (BCYE) agar supplemented with antibiotics to prevent overgrowth of other bacteria. Cultures must be held for 14 days and the reported sensitivity of culture is low at 25-80%. Many hospital laboratories do not offer Legionella culture.

DFA staining of respiratory specimens has a low sensitivity of 25 to 75% because large numbers of organisms are required for visualization. Staining with monoclonal antibody is superior to polyclonal antibody for detecting L. pneumophila in respiratory specimens because background fluorescence and cross-reactivity with non-Legionella bacteria are reduced.

Serologic and urine antigen tests are also available, when respiratory specimens are difficult to collect. Serological testing for Legionella antibody is useful in epidemiologic studies but is less helpful in making a rapid diagnosis of Legionnaires' disease. A single elevated titer does not confirm a case of Legionnaires' disease because elevated titers have been detected up to 16% of healthy adults. Serologic testing should include both IgM and IgG antibodies obtained both acutely and during convalescence. The definitive criterion for serologic diagnosis is a fourfold rise in antibody titer. Antibody response may not be detectable until one to three months after the onset of illness. Convalescent specimen should be collected 8 to 12 weeks after the onset of infection. Some patients never demonstrate a fourfold rise in antibody titer. Serology sensitivity ranges between 40 and 60% with a specificity of 96 to 99%.

The urine antigen test detects a specific soluble antigen present in the urine of patients with Legionella infection and is the test of choice for diagnosis of early infection. It detects only Legionella pneumophila serogroup 1, which accounts for 90% of community acquired cases of legionellosis. Sensitivity is 80% and specificity is 97 to 100%. Antigen excretion may begin as early as 3 days after onset of symptoms and persist for up to 1 year afterward. The test remains positive for several weeks following antibiotic therapy.

Legionella PCR testing of respiratory specimens is available through a reference laboratory, and is the best alternative when this diagnosis is highly suspected. Legionella PCR does not differentiate between Legionella species. PCR detects a specific sequence located within the Legionella 5S rRNA gene. A positive PCR result indicates the presence of a Legionella species DNA, which may be due to Legionella infection. False positive results are theoretically possible since Legionella is an aquatic organism that can contaminate water. False negative results may occur due to inhibition of PCR. PCR is not useful in monitoring treatment because DNA remains detectable for a long time after disease resolution.

Specimen requirement for serologic testing is one red top tube of blood.  Specimen requirement for the urine antigen test is 1.0 mL of urine from a random collection. No urine preservatives should be used, and the specimen should be refrigerated after collection. PCR can be performed on sputum, tracheal secretions or aspirates, bronchial washing or aspirate, bronchoalveolar lavage, pleural fluid or fresh lung tissue.


Dooling KL, Toews KA, Hicks LA, et al. Active bacterial core surveillance for legionellosis—United States, 2011–2013. MMWR Morb Mortal Wkly Rep 2015;64:1190–3.

Soda EA, Barskey AE, Shah PP, et al. Vital Signs: Health Care–Associated Legionnaires’ Disease Surveillance Data from 20 States and a Large Metropolitan Area — United States, 2015. MMWR Morb Mortal Wkly Rep 2017;66:584–589. DOI:

Diederen BM, Kluytmans JA, Vandenbroucke-Grauls CM, Peeters MF: Utility of real-time PCR for diagnosis of Legionnaires' disease in routine clinical practice. J Clin Microbiol 2008;46(2):671-677

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