Legionella pneumophila


Indications for Testing

Laboratory Testing

  • Legionellosis resources (CDC)
  • Perform routine microbiology testing only for pathogens that would alter empirical decisions (recommendation from Infectious Diseases Society of America and American Thoracic Society, 2007)
  • Nonspecific testing may reveal the following
    • Serum sodium – hyponatremia
    • Serum phosphate – hypophosphatemia
    • Urine analysis – hematuria
    • CBC – thrombocytopenia (common)
    • Liver function studies – elevated transaminases
  • Consider testing for other common agents of community-acquired pneumonia – Streptococcus pneumoniae, Chlamydophila pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae
  • Rapid testing may aid in early initiation of treatment
    • Urine antigen testing (ELISA)  
      • Relatively short turnaround time (TAT) – point of care for some assays
      • Urine antigen testing more sensitive than DFA and highly specific for serogroup 1
        •   Sensitivity depends on severity of disease, presence of serogroup 1
      • Detects only serogroup 1 (most common disease-causing serogroup)
      • Antigens or antibodies may be detected as early as 1 day after symptom onset; positivity of test may persist >60 days
        • Cannot be used for test of cure
      • For negative test early in illness and high pretest probability – repeat test in 2-3 days
    • Respiratory specimen
      • DFA – highly insensitive compared to PCR and culture
        • Can detect various serogroups
        • Technically demanding; requires large respiratory specimen
    • PCR – good sensitivity; low rate of false positives
  • Culture – gold standard
    • Requires sputum or bronchoalveolar lavage (BAL)
    • May be used to confirm PCR results
    • Minimum of 48 hours TAT – fastidious with average culture taking 2-5 days for growth 
    • Genotyping isolates may help identify source of outbreak
  • Serological testing
    • May be used for retrospective diagnosis – seroconversion may take several weeks

Imaging Studies

  • Chest x-ray
    • Rapidly progressive asymmetrical infiltrates are common
    • Infiltrates may initially enlarge concurrent with clinical improvement while on appropriate antimicrobial therapy
    • Cavitation may occur – most common in immunocompromised patients

Differential Diagnosis

Clinical Background

Legionellosis refers to two clinical syndromes caused by bacteria of the genus Legionella – Legionnaires’ disease and Pontiac fever. Legionnaires’ disease is considered an atypical pneumonia.


  • Incidence – >10,000 annually in U.S.
  • Age – usually 45-64 years; rare cause of pneumonia in children
  • Sex – M>F; 2-3:1
  • Transmission
    • Majority are sporadic
    • More common in summer months – outbreaks of pneumonia have occurred after inhalation of contaminated water droplets from water sources such as evaporative coolers, hot tubs, heating systems
    • Person-to-person transmission has not been shown
    • Nosocomial cases are not uncommon
      • Sources of infection are often hospital showers, drinking fountains, and respiratory-therapy equipment


  • Fastidious gram-negative coccobacillus
    • Widespread in nature but live mainly in freshwater bodies
    • Parasitizes freshwater free living amoeba in the environment and pulmonary macrophages in humans
  • >50 species compose at least 70 different serogroups of Legionellae
  • L. pneumophila is major pathogen and most commonly encountered member of group
    • Serogroup 1 is most common infecting type

Risk Factors

  • Adults
    • Older age (average ~55 years)
    • Tobacco use
    • Underlying chronic disease (eg, chronic obstructive pulmonary disease, diabetes, renal failure)
    • Compromised immune system (immunosuppressive therapy or T-cell dysfunction, including cancer)
    • Recent surgery
    • Alcoholism
    • Long-term use of breathing machines (ventilators)
  • Children – prematurity, immunocompromised state, bronchopulmonary dysplasia

Clinical Presentation

  • Legionnaires' disease
    • Incubation period of 2-15 days
      • Prodromal phase of headache and myalgia progressing to fever with chills
        • Often have relative bradycardia instead of fever
      • Productive cough, possibly with hemoptysis 
      • Chest pain 
      • Shortness of breath
    • Symptoms may suggest pneumococcal pneumonia
    • Gastrointestinal symptoms common – primarily loose stools, diarrhea and/or abdominal pain
    • Systemic involvement common – kidney, liver and central nervous system dysfunction
      • Central nervous system – headache, lethargy, confusion, ataxia, encephalopathy 
    • Case fatality rate – 5-30%
      • Greater risk of death exists for elderly and immunocompromised  
  • Pontiac fever
    • Self-limiting, febrile illness with non-pneumonic, influenza-like illness characterized by myalgia, malaise and fever
      • Usually affects healthy individuals

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Legionella pneumophila Antigen, Urine 0070322
Method: Qualitative Enzyme-Linked Immunosorbent Assay

Rapid diagnostic test with good sensitivity and high specificity

Sensitivity – 70%  

Specificity – 100%

Only detects antigens to L. pneumophila serogroup 1; however, this serogroup causes >70% of the Legionnaires’ disease cases  
Legionella Species by Qualitative PCR 2010125
Method: Qualitative Polymerase Chain Reaction

Relatively rapid diagnostic test

Aids in diagnosing etiology of pneumonia when Legionella is suspected

Detects and speciates L. pneumophila

Sensitivity – 80%

Specificity – 100%

Nucleic acid from other Legionella species will be detected by this test but cannot be differentiated

Only for respiratory secretions

Negative result does not rule out the presence of PCR inhibitors in patient specimen or test-specific nucleic acid in concentrations below the level of detection by this test

Legionella Species, Culture 0060113
Method: Culture/Identification

Gold standard test; detects L. pneumophila and other Legionella species

May be useful in determining source of outbreaks

Sensitivity – 20-95%

Specificity – 100%

Requires 3-5 days

Most sensitive in severe disease

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Legionella pneumophila DFA 2004598
Method: Direct Fluorescent Antibody Stain

Not a first-line test for diagnosis of Legionella infection

For Legionella diagnosis use culture, PCR, or antigen (urine)

Negative test does not rule out Legionella infection

Sensitivity – 25-75%

Sodium, Plasma or Serum 0020001
Method: Quantitative Ion-Selective Electrode

Nonspecific testing

Phosphorus, Inorganic, Plasma or Serum 0020028
Method: Quantitative Spectrophotometry

Nonspecific testing

Urinalysis, Complete 0020350
Method: Reflectance Spectrophotometry/Microscopy

Nonspecific testing

CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential

Nonspecific testing

May be useful in distinguishing bacterial from viral etiologies

Hepatic Function Panel 0020416
Method: Quantitative Enzymatic/Quantitative Spectrophotometry

Nonspecific testing

Panel includes albumin; ALP; AST; ALT; bilirubin, direct; protein, total; and bilirubin, total

Legionella pneumophila Antibody (Type 1), IgM by IFA 0050273
Method: Semi-Quantitative Indirect Fluorescent Antibody

Epidemiologic purposes only

Sensitivity – 40-60%  

Specificity – 96-99%

Legionella pneumophila Antibody (Types 1-6), IgM by IFA 0050274
Method: Semi-Quantitative Indirect Fluorescent Antibody

Epidemiologic purposes only

Sensitivity – 40-60%  

Specificity – 96-99%

Legionella pneumophila Antibody (Types 1-6), IgG by IFA 0050365
Method: Semi-Quantitative Indirect Fluorescent Antibody

Epidemiologic purposes only

Sensitivity – 40-60%  

Specificity – 96-99%

Legionella pneumophila Antibody (Type 1), IgG by IFA 0050376
Method: Semi-Quantitative Indirect Fluorescent Antibody

Epidemiologic purposes only

Sensitivity – 40-60%  

Specificity – 96-99%

Chlamydia pneumoniae by PCR 0060715
Method: Qualitative Polymerase Chain Reaction
Mycoplasma pneumoniae by PCR 0060256
Method: Qualitative Polymerase Chain Reaction
Streptococcus pneumoniae Antigen, Urine 0060228
Method: Qualitative Immunochromatography