From the Guidelines
The Legionella urinary antigen test is a valuable diagnostic tool for detecting Legionnaires' disease, providing results within 15 minutes to a few hours, and should be used in patients with moderate to severe pneumonia, particularly those with risk factors such as immunocompromise, recent travel, or exposure to water systems like cooling towers or hot tubs, as recommended by the most recent guidelines 1.
Key Points
- The test specifically identifies Legionella pneumophila serogroup 1, which causes approximately 70-80% of Legionella infections.
- The test has high specificity (95-100%) but moderate sensitivity (70-80%), meaning a positive result strongly indicates infection while a negative result doesn't rule it out.
- False positives can occur in patients with rheumatoid factor or other conditions, and the antigen can remain detectable for weeks after treatment, limiting its use for monitoring response.
- If Legionella is strongly suspected despite a negative urinary antigen, additional testing like sputum culture or PCR should be performed.
- Treatment should not be delayed while awaiting results if Legionnaires' disease is suspected; empiric therapy with azithromycin 500mg daily or levofloxacin 750mg daily for 5-10 days should be initiated promptly, as supported by previous studies 2, 3, 4, 5.
Considerations
- The test is most useful in patients with moderate to severe pneumonia, particularly those with risk factors such as immunocompromise, recent travel, or exposure to water systems like cooling towers or hot tubs.
- The test should be used in conjunction with clinical judgment and other diagnostic tests to guide treatment decisions.
- The availability of a culture isolate of Legionella can improve the likelihood that an environmental source of Legionella can be identified and remediated, as noted in previous studies 5.
From the Research
Utility of Legionella Urinary Antigen
- The Legionella urinary antigen test is a useful diagnostic tool for detecting Legionnaires' disease, with a pooled sensitivity of 0.74 and specificity of 0.991 6.
- The test is recommended for all patients with community-acquired pneumonia, especially those with ambulatory pneumonia and hospitalized children 7.
- The urinary antigen test has been found to be a reliable method for detecting Legionella pneumophila serogroup 1, with a sensitivity of 0.86 and specificity of 1.00 8.
- The test is particularly useful in patients with severe community-acquired pneumonia, where testing for Legionella spp. is recommended at hospital admission 9.
- However, the clinical utility of the test may be limited in low-prevalence centers, where the cost of diagnosing each case of Legionella pneumonia by urinary antigen testing may be high 10.
Diagnostic Accuracy
- The overall pooled sensitivity and specificity of urinary antigen tests for legionellosis were found to be 0.79 and 1.00, respectively 8.
- Subpopulation analysis revealed that the accuracy of urinary antigen tests for sensitivity and specificity for Legionella pneumophilia serogroup 1 was 0.86 and 1.00, respectively 8.
- The diagnostic accuracy of urinary antigen tests may be influenced by advances in test technology and epidemiological changes in the frequency of Legionella species that cause legionellosis 8.
Clinical Implications
- The use of urinary antigen tests can help guide antibiotic therapy, with levofloxacin or azithromycin recommended as first-line therapy for Legionnaires' disease 7, 6, 9.
- Inadequate or delayed antibiotic treatment in Legionella pneumonia has been associated with a worse prognosis, highlighting the importance of early diagnosis and treatment 9.
- Further studies are needed to assess the usefulness of diagnostic tests regarding clinical outcomes, as well as randomized trials comparing fluoroquinolones and macrolides or combination therapy 9.