Legionellosis is a bacterial disease caused by Legionella that can present as either Legionnaires disease or Pontiac fever. Legionnaires disease is a severe form of pneumonia that often requires hospitalization. Pontiac fever is a milder disease that is often self-limiting. Legionella is naturally found in freshwater environments, but can become a health concern when it grows and spreads in human-made water systems such as hot tubs and large plumbing systems. Inhalation of Legionella-containing water droplets can cause infection, especially in people at increased risk (eg, immunocompromised individuals). Laboratory testing such as polymerase chain reaction (PCR), culture, and urinary antigen testing is used to diagnosis legionellosis.
Quick Answers for Clinicians
People who are ≥50 years old, current or former smokers, or immunocompromised, and those with a chronic lung disease, cancer, or other underlying illness such as diabetes or kidney failure are at increased risk for legionellosis. Additionally, people who have recently traveled, with an overnight stay outside of their homes (including a stay in a healthcare facility), and people who have had recent exposure to hot tubs are at increased risk for legionellosis.
The American Thoracic Society (ATS) and the Infectious Diseases Society of America (IDSA) do not recommend routinely testing for Legionella in adults with community-acquired pneumonia (CAP), except in cases indicated by epidemiologic factors (eg, recent outbreak or travel) or in cases of severe CAP. In adults with severe CAP, a combination of urinary antigen testing and culture or nucleic acid-based testing such as polymerase chain reaction (PCR) is recommended.
Indications for Testing
Laboratory testing for Legionnaires disease should be considered in patients who have pneumonia (often initially confirmed with a chest x-ray) and meet one or more of the following conditions:
- Lack of response to outpatient antibiotic treatment for community-acquired pneumonia
- Severe pneumonia (particularly if intensive care is required)
- Immunocompromised status
- History of recent travel (within 10 days of disease onset)
- Risk for Legionnaires disease with healthcare-associated pneumonia
- Exposure to a setting with a Legionnaires disease outbreak
Testing for healthcare-associated Legionnaires disease is especially important in settings with a confirmed diagnosis within the last 12 months, positive environmental Legionella testing in the past 2 months, or current changes in water quality that may lead to Legionella growth.
Testing for Pontiac fever is generally not indicated because the disease is mild and resolves without treatment. However, if there is a possibility that the patient has been exposed to Legionella at the same time or place as another patient with laboratory-confirmed legionellosis, testing can be considered.
There are many laboratory tests available for the diagnosis of Legionella. All tests have advantages and disadvantages. Generally, PCR, urinary antigen testing, and culture are preferred, although other methodologies may be useful in certain situations. The table below compares the various testing strategies.
Can be performed on wide range of sample types, including sputum and lung tissue (although lower respiratory samples are preferred)
Detects all clinically relevant species and serogroups
Fast turnaround time (<2 days)
High specificity and sensitivity
Assays may vary by laboratory
False-positive results are possible because PCR can detect nonviable Legionella
Not affected by antibiotic treatment
Provides retrospective evidence of Legionella infection
Does not detect all species or serogroups, so false-negative results are possible
Does not enable comparison between clinical and environmental isolates
Detects all species and serogroups of Legionella
Enables comparison of clinical and environmental isolates to help determine the source of an outbreak
Technically difficult, requires specialty media
Long turnaround time (>5 days)
Unreliable after initiation of antibiotic treatment
Can be performed on lung tissue in addition to respiratory samples
Detects multiple species and serogroups
Can be used as a confirmatory test for suspected Legionella colonies isolated from culture
Sensitivity is highly dependent on patient population and specimen type
Not recommended for initial diagnosis of legionellosis
|Serology||Detects multiple species and serogroups||
Requires paired sera collections at onset of symptoms and after 3-6 wks for accurate analysis
Long turnaround time (due to need for paired sera collection)
aPositive result is considered confirmatory.
DFA, direct fluorescent antibody
ARUP Laboratory Tests
Qualitative Enzyme-Linked Immunosorbent Assay
Semi-Quantitative Enzyme-Linked Immunosorbent Assay
CDC - Legionella Clinical Features
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Legionella (Legionnaires' disease and Pontiac fever): clinical features. [Last reviewed: Jan 2020; Accessed: Feb 2020]
CDC - Legionella Diagnostic Testing
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Legionella (Legionnaires' disease and Pontiac fever): diagnosis, treatment, and prevention. [Last reviewed: Jan 2020; Accessed: Feb 2020]
CDC - Legionella Causes and Common Sources
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Legionella (Legionnaires' disease and Pontiac fever): causes, how it spreads, and people at increased risk. [Last reviewed: Jan 2020; Accessed: Feb 2020]
CDC - Legionella Disease Specifics
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Legionella (Legionnaires' disease and Pontiac fever): disease specifics. [Last reviewed: Apr 2018; Accessed: Feb 2020]
Metlay JP, Waterer GW, Long AC, et al. Diagnosis and treatment of adults with community-acquired pneumonia. An official clinical practice guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med. 2019;200(7):e45-e67.
CDC - Legionella Diagnosis
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Legionella (Legionnaires' disease and Pontiac fever): diagnosis, treatment, and complications. [Last reviewed: Apr 2018; Accessed: Feb 2020]
Pierre DM, Baron J, Yu VL, et al. Diagnostic testing for Legionnaires' disease. Ann Clin Microbiol Antimicrob. 2017;16(1):59.