What is the best diagnostic test for Legionella, particularly in older adults or individuals with underlying health conditions?

Medical Advisory BoardAll articles are reviewed for accuracy by our Medical Advisory Board
Educational purpose only • Exercise caution as content is pending human review
Article Review Status
Submitted
Under Review
Approved

Last updated: February 1, 2026 • View editorial policy

Personalize

Help us tailor your experience

Which best describes you? Your choice helps us use language that's most understandable for you.

Best Diagnostic Test for Legionella

Urinary antigen testing is the best diagnostic test for Legionella, particularly in older adults and those with underlying health conditions, as it provides rapid results within minutes, has high specificity (approaching 100%), and its early use has been shown to improve outcomes and reduce mortality in Legionnaires' disease. 1

Primary Recommendation: Urinary Antigen Test

The urinary antigen test is currently the most helpful rapid diagnostic test for Legionella infection and should be the first-line test. 1 The immunochromatographic format (ICT) is particularly well-suited for individual specimens and produces results within minutes, making it ideal for clinical decision-making. 1

Performance Characteristics

  • Sensitivity varies by disease severity and specimen concentration:

    • Unconcentrated urine: 63.7-66.6% sensitivity 1
    • Concentrated urine: 86.6-88.8% sensitivity 1
    • Immunochromatographic assay on concentrated specimens: 97.2% sensitivity 1
    • Severe Legionnaires' disease: 88-100% sensitivity 1
    • Mild disease: 40-53% sensitivity 1

  • Specificity is excellent: approaching 100% 2

  • Recent meta-analysis (2022) confirms: pooled sensitivity of 79% (95% CI: 0.71-0.85) and specificity of 100% (95% CI: 0.99-1.00) overall, with sensitivity increasing to 86% specifically for L. pneumophila serogroup 1 2

Clinical Impact on Outcomes

Since the introduction of urinary antigen testing, early diagnosis and treatment has improved outcomes and reduced case fatality rates in Legionellosis outbreaks. 1 This is particularly critical for older adults and those with underlying conditions, as the degree of urinary antigen test positivity correlates with disease severity. 1

Timing Considerations

  • Collect urine as early as possible in the disease course 3
  • The test may be negative during the first 5 days of illness in some patients 1
  • Antigen can be detected as early as 2 days after symptom onset 3
  • The test can remain positive for 6-60 days after onset 1, 3

Test Selection

Among available urinary antigen tests, the Binax NOW test demonstrates superior sensitivity compared to other ICT assays, detecting L. pneumophila serogroup 1 in 74.3% of cases versus 61.5% for SAS and 41% for Uni-Gold. 1, 4 Newer tests like Meridian TRU Legionella show comparable performance (77% sensitivity) to Binax NOW. 5

Critical Limitation: Serogroup Coverage

The major caveat is that current urinary antigen tests primarily detect L. pneumophila serogroup 1 only. 1 Other serogroups of L. pneumophila and other Legionella species are not reliably detected, though cross-reactions occasionally occur. 1 This leads to potential under-recognition of non-serogroup 1 infections, which is associated with more severe clinical courses. 1

Supplementary Testing Strategy

When to Add Culture

Culture remains the gold standard and should be performed alongside urinary antigen testing because it can identify all Legionella species and serogroups, not just serogroup 1. 6 However, culture is slow (3-4 days) and the urinary antigen test is frequently the first positive laboratory result. 1

Alternative Specimen Testing

  • Pleural fluid: If a parapneumonic effusion is present and pleural fluid is obtained, test it with the ICT assay (sensitivity 79%, specificity 94%), especially if urinary antigen is negative 1
  • Respiratory specimens: Direct fluorescent antibody testing has poor sensitivity (22-75%) and is not recommended as a primary test 1

Role of Serology

Serology is NOT useful for individual patient management as it requires paired sera separated by weeks and cannot guide acute treatment decisions. 1 Serology is more appropriate for epidemiological studies than clinical care. 1

Practical Algorithm for Older Adults and High-Risk Patients

  1. Order urinary antigen test immediately when Legionella is suspected (severe CAP, epidemiological risk factors) 1
  2. Use concentrated urine specimens when possible to maximize sensitivity 1
  3. Simultaneously send respiratory specimens for culture on selective media (BYCE with antibiotics) to detect non-serogroup 1 infections 6
  4. If pleural effusion present, test pleural fluid with ICT assay 1
  5. Do not delay empiric antibiotic therapy while awaiting test results in severely ill patients

Common Pitfalls to Avoid

  • Do not rely solely on urinary antigen testing as it misses non-serogroup 1 infections 1
  • Do not wait for serology results to guide acute management 1
  • Do not dismiss Legionella if early urinary antigen is negative—repeat testing after 24-48 hours if clinical suspicion remains high 1, 3
  • Do not use respiratory antigen tests as they lack adequate sensitivity 1

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

4
Research

Evaluation of three immunochromatographic assays for detection of Legionella pneumophila serogroup 1 antigen in urine samples.

Revista espanola de quimioterapia : publicacion oficial de la Sociedad Espanola de Quimioterapia, 2009

5
Research

Evaluation of Meridian TRU Legionella®, a new rapid test for detection of Legionella pneumophila serogroup 1 antigen in urine samples.

European journal of clinical microbiology & infectious diseases : official publication of the European Society of Clinical Microbiology, 2013

6
Research

Diagnostic testing for Legionnaires' disease.

Annals of clinical microbiology and antimicrobials, 2017

Related Questions

What is the recommended procedure for collecting a Legionnaires' (Legionella) test?
What is the utility of the Legionella urinary antigen test (Legionella pneumophila antigen) in diagnosing Legionnaires' disease?
What is the recommended workup and treatment for Legionnaires' disease?
Can sulfasalazine (a medication used to treat inflammatory bowel disease and rheumatoid arthritis) cause weight loss in patients, particularly those with a history of gastrointestinal issues or decreased appetite?
How to manage epigastric pain in a patient with Glucose-6-phosphate dehydrogenase (G6PD) deficiency?
What is the role of hydrocortisone (cortisol) in the treatment of patients with heart failure, particularly those with acute decompensated heart failure or signs of significant inflammation?
What is the best approach for managing abdominal pain in an emergency room (ER) patient with liver failure, considering their compromised liver function and potential for bleeding or infection?
How to place a nasogastric tube (NGT) in a patient with a history of vomiting without inducing vomiting?
What is the step-wise management approach for patients with heart failure, including lifestyle modifications, medical therapy (ACE inhibitor (Angiotensin-Converting Enzyme inhibitor) or ARB (Angiotensin Receptor Blocker)), device therapy (ICD (Implantable Cardioverter-Defibrillator), CRT (Cardiac Resynchronization Therapy)), and advanced heart failure treatments, as per the most recent AHA (American Heart Association)/ACC (American College of Cardiology)/HFSA (Heart Failure Society of America) guidelines?

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

Have a follow-up question?

Our Medical A.I. is used by practicing medical doctors at top research institutions around the world. Ask any follow up question and get world-class guideline-backed answers instantly.

Ask Question