Testing for Legionnaires' disease involves a combination of laboratory tests and clinical evaluation. The primary and preferred diagnostic tests are:
- Urinary Antigen Test (UAT) - Detects Legionella pneumophila serogroup 1 antigen in urine. It is widely used because it is quick and non-invasive but detects primarily one type of Legionella.
- Culture or Molecular Testing of Lower Respiratory Specimens - Such as sputum, bronchoalveolar lavage, or tracheal aspirate. These tests can detect a range of Legionella species and serogroups and can be more sensitive, especially after antibiotic treatment starts.
- Other tests may include blood tests looking for antibodies or PCR (Polymerase Chain Reaction) testing for Legionella DNA.
Chest X-rays are also used to diagnose pneumonia that might suggest Legionnaires' disease but do not identify the bacteria. Best practice is to collect urine and lower respiratory specimens together for testing, ideally before starting antibiotics, but treatment should not be delayed to collect samples. In clinical settings, diagnosis usually starts with suspicion of pneumonia and is confirmed with the above tests. Summary:
Test| Specimen| Detects| Notes
---|---|---|---
Urinary Antigen Test (UAT)| Urine| Legionella pneumophila serogroup 1 antigen|
Fast, common, non-invasive, limited to main type
Culture| Sputum, lung tissue| Wide range of Legionella species| Gold standard
for outbreak source comparison
PCR (Molecular test)| Respiratory specimens| Legionella DNA| High sensitivity,
rapid, detects broader types
Blood Serology| Blood| Antibodies (past infection)| Not used alone for acute
diagnosis
Chest X-ray| Imaging| Pneumonia detection| Supports pneumonia diagnosis, not
specific to Legionella
These testing methods together confirm Legionnaires' disease by identifying
the causative bacteria. References:
CDC guidance on Legionella testing, sensitivity, and specimen collection.
Mayo Clinic, Cleveland Clinic, MedlinePlus on test types and procedures
