Evaluation for Legionnaires' Disease
The recommended diagnostic approach for Legionnaires' disease is urinary antigen testing combined with culture of respiratory secretions on selective media, with urinary antigen testing being the most rapid and clinically useful initial test. 1
Clinical Indications for Testing
Testing for Legionella is particularly indicated in the following scenarios:
- Patients hospitalized with enigmatic pneumonia
- Severe pneumonia requiring ICU admission
- Immunocompromised patients with pneumonia
- During Legionella outbreaks
- Patients failing to respond to β-lactam antibiotics
- Patients over 40 years of age with pneumonia
- Patients with epidemiological risk factors 2, 1
Epidemiological Risk Factors
- Recent travel with overnight stays
- Exposure to spas or hot tubs
- Recent changes in domestic plumbing
- Underlying conditions: renal failure, hepatic failure, diabetes, systemic malignancy 2
Diagnostic Tests
1. Urinary Antigen Testing (First-line)
- Detects L. pneumophila serogroup 1 (responsible for 80-95% of community-acquired cases)
- Sensitivity: 63.7-66.6% in unconcentrated urine, 86.6-88.8% in concentrated urine
- Can detect antigen 6-14 days after infection onset
- Results available within hours
- Limitations: Only detects L. pneumophila serogroup 1 2, 1
2. Culture of Respiratory Secretions (Gold Standard)
- Detects all Legionella species and serogroups
- Specimens: sputum, bronchoscopy samples, or endotracheal aspirates
- Requires selective media (buffered charcoal yeast extract with antibiotics)
- Technically demanding and requires 3-7 days for results
- Should be performed even if urinary antigen is positive to identify specific strain 2, 1
3. PCR Testing
- Higher sensitivity than urinary antigen testing
- Can detect non-serogroup 1 infections
- Not FDA-cleared but increasingly available
- Particularly useful in outbreak investigations 1
4. Serology (Less Useful for Acute Diagnosis)
- Four-fold rise in L. pneumophila antibody titer to ≥1:128 in paired acute and convalescent sera
- Not useful for initial management decisions due to delayed results
- A single elevated titer is not diagnostic (found in 1-16% of healthy adults) 2
Specimen Collection
- Urine: Collect for antigen testing (consider concentration to improve sensitivity)
- Respiratory specimens: Obtain before antibiotic administration if possible
- Blood cultures: Should be collected in all patients with suspected pneumonia
- Pleural fluid: If present, should be aspirated and cultured 2, 1
Diagnostic Algorithm
- Collect urine for Legionella antigen testing and respiratory specimens for culture
- Initiate PCR testing if available, especially for severe cases
- Begin appropriate antibiotic therapy immediately if Legionnaires' disease is suspected, without waiting for test results
- Continue treatment even if diagnostic tests are negative when clinical suspicion is high 2, 1
Important Caveats
- No single test detects all cases of Legionnaires' disease; a combination of tests increases diagnostic yield 1
- Clinical features alone cannot reliably distinguish Legionnaires' disease from other causes of pneumonia, though some features (high fever, hyponatremia, CNS manifestations, elevated lactate dehydrogenase) may suggest the diagnosis 2
- Early treatment significantly improves outcomes and reduces mortality; do not delay treatment while awaiting test results 2
- Treatment for legionnaires' disease should be given or continued even if test results are negative when clinical suspicion is high 2
Treatment Considerations
For hospitalized patients with confirmed or suspected Legionnaires' disease:
- First-line: Azithromycin or a respiratory fluoroquinolone (levofloxacin, moxifloxacin)
- Duration: 10-21 days (shorter for azithromycin due to long half-life)
- Initiate treatment as soon as possible, as delay is associated with increased mortality 2
For non-hospitalized patients with milder disease:
- Options include erythromycin, doxycycline, azithromycin, clarithromycin, or a fluoroquinolone 2