Diagnosis, Treatment, and Prevention
The preferred diagnostic tests for Legionnaires’ disease are culture of lower respiratory secretions (e.g., sputum, bronchoalveolar lavage) on selective media and the Legionella urinary antigen test. Serological assays can be nonspecific and are not recommended in most situations. Best practice is to obtain both sputum culture and the urinary antigen test concurrently. Sputum should ideally be obtained prior to antibiotic administration, but antibiotic treatment should not be delayed to facilitate this process. The urinary antigen test can detect Legionella infections in some cases for days to weeks after treatment.
Listed below are indications that warrant testing patients with pneumonia for Legionnaires’ disease:
- Patients who have failed outpatient antibiotic treatment for community-acquired pneumonia
- Patients with severe pneumonia, in particular those requiring intensive care
- Immunocompromised patients with pneumonia*
- Patients with a travel history (patients who have traveled away from their home within 10 days before the onset of illness)
- All patients with pneumonia in the setting of a Legionnaires’ disease outbreak
- Patients at risk for Legionnaires’ disease with healthcare-associated pneumonia (pneumonia with onset ≥ 481 hours after admission)
The Council of State and Territorial Epidemiologists (CSTE) position statement outlines case definitions for confirmed and suspected legionellosis. Read the position statement.
*May also consider testing for Legionnaires’ disease in patients with other risk factors for Legionnaires’ disease as listed here: https://www.cdc.gov/legionella/clinicians/disease-specifics.html
Testing for healthcare-associated Legionnaires’ disease is especially important if any of the following are identified in a health care facility:
- Other patients with healthcare-associated Legionnaires’ disease diagnosed in the past 12 months
- Positive environmental tests for Legionella in the past 2 months
- Current changes in water quality that may lead to Legionella growth (such as low chlorine levels or nearby construction)
Timely reporting of healthcare-associated cases ensures that steps can be taken to protect these highly susceptible populations.
The majority of recognized Legionnaires’ disease outbreaks are associated with travel (hotels, resorts, cruise ships) or healthcare settings (hospitals, long-term care facilities).2 Approximately 10–15% of all reported cases of Legionnaires’ disease occur in people who have traveled during the 10 days before symptom onset. Outbreaks among travelers can be difficult to detect because of
- The low attack rate
- The time interval between exposure and symptom onset
- Dispersal of people from the source of the outbreak
- Underdiagnosis of cases
Therefore, collecting and reporting information about overnight travel is important. See Healthcare Investigation Resources for more information about healthcare exposures.
Isolation of Legionella on media that supports growth of Legionella (i.e., Buffered Charcoal Yeast Extract [BCYE] agar) from lower respiratory secretions, lung tissue, pleural fluid, or a normally sterile site is confirmatory and an important method for diagnosis. It can detect other Legionella species and serogroups that the urinary antigen test does not.
- Unlike the urinary antigen test, culturing specimens from patients can detect all species and serogroups of Legionella
- Isolating Legionella from clinical specimens helps investigators identify where exposure occurred and prevent additional cases
Comparing clinical and environmental isolates using serologic and molecular techniques can help identify the source in Legionnaires’ disease outbreak investigations. Because Legionella commonly occurs in the environment, clinical isolates can help interpret the findings of an environmental investigation.
Urinary Antigen Test
The most commonly used laboratory test for diagnosis of Legionnaires’ disease is the urinary antigen test, which detects a molecule of the Legionella bacterium in urine. If the patient has pneumonia and the test is positive, then you should consider the patient to have Legionnaires’ disease. The test can remain positive for a few weeks after infection, even with antibiotic treatment. The urinary antigen test detects the most common cause of Legionnaires’ disease, L. pneumophila serogroup 1. However, all species and serogroups of Legionella are potentially pathogenic, so a patient with a negative urinary antigen result could have Legionnaires’ disease caused by other Legionella species and serogroups.
Sensitivity varies depending on the quality and timing of clinical specimen collection, as well as technical skill of the laboratory worker performing the test. The table below provides general ranges for the sensitivity and specificity of each diagnostic test.
|Test||Sensitivity (%)||Specificity (%)|
|Urinary antigen for L. pneumophila serogroup 1 (Lp1)||70–100||95–100|
|Direct Fluorescent Antibody (DFA) Stain||25–75||≥95|
|Polymerase Chain Reaction (PCR)3||95–99||>99|
1 Cross reactions with other species and serogroups have been documented.
2 CDC labs do not perform serologic testing for legionellosis diagnosis due to inherent challenges in obtaining appropriate specimens. It is important to note that because paired sera are required, results are delayed and thus may not be useful for acute case diagnosis or during active outbreak investigations.
3 Avni T, Bieber A, Green H, et al. Diagnostic accuracy of PCR alone and compared to urinary antigen testing for detection of Legionella spp.: A systematic review. J Clin Micro. 2016;54(2):401–11.
1 Cross reactions with other species and serogroups have been documented.
2 Yu V, Plouffe JF, Pastoris MC, et al. Distribution of Legionella species and serogroups isolated by culture in patients with sporadic community-acquired legionellosis: An international collaborative survey. J Infect Dis. 2002;186:127–8.
3 CDC labs do not perform serologic testing for legionellosis diagnosis due to inherent challenges in obtaining appropriate specimens. It is important to note that because paired sera are required, results are delayed and thus may not be useful for acute case diagnosis or during active outbreak investigations.
4 Laboratories can develop, adopt, or use existing assays (known as laboratory developed tests) for in vitro diagnostic purposes. Labs do this by establishing and validating test performance under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) requirements or equivalent regulations.
5 Currently the U.S. Food and Drug Administration has cleared at least one nucleic acid-based molecular assay for Legionella pneumophila detection.
If your patient has Legionnaires’ disease, please see the most recent IDSA-ATS guidelines for treatment of community-acquired pneumonia pdf icon[46 pages]external icon and the most recent IDSA-ATS guidelines for treatment of hospital-acquired pneumonia pdf icon[51 pages]external icon. Note that first line treatment, however, does not always include Legionella-directed antibiotics (e.g., macrolides and respiratory fluoroquinolones). While it is preferred that you obtain diagnostic testing before antibiotic administration, antibiotic treatment should not be delayed to facilitate this process.
If your patient has Pontiac fever, antibiotic treatment should not be prescribed. It is a self-limited illness that does not benefit from antibiotic treatment. Patients usually recover within 1 week.
Minimizing Legionella growth in complex building water systems and devices, including potable water, hot tubs, decorative fountains, and cooling towers, is key to preventing infection. Timely identification and reporting of legionellosis cases is also important because this allows public health officials to quickly identify and stop potential clusters and outbreaks by linking new cases to previously reported ones.
Report cases and outbreaks of Legionnaires’ disease to your local or state health department. Refer to your state health department website to find more information about how to contact your state or local health department.
- Legionnaires’ Disease Fact Sheet for Patients
English pdf icon[1 page] | Spanish pdf icon[1 page]
- Legionnaires’ Disease Fact Sheet for Clinicians pdf icon[2 pages]
- Legionnaires’ Disease on Rise in US—2016 Updateexternal icon
- Kalil AC, Metersky ML, Klompas M, et al. Management of adults with hospital-acquired and ventilator-associated pneumonia: 2016 clinical practice guidelines by the Infectious Diseases Society of America and the American Thoracic Society pdf icon[51 pages]external icon. Clin Infect Dis. 2016;63(5):e61–111.
- Garrison LE, Kunz JM, Cooley LA, et al. Vital Signs: Deficiencies in environmental control identified in outbreaks of Legionnaires’ disease — North America, 2000–2014. MMWR Morb Mortal Wkly Rep. 2016;65(22):576–84.