Legionella Tests
Reviewed
Legionella tests look for bacteria that cause Legionnaires' disease, a severe form of pneumonia. The tests are used to diagnose the infection so that prompt antibiotic treatment can be given. Testing is usually done on urine or sputum samples; sometimes a bronchoscopy with bronchoalveolar lavage is needed to obtain a lung sample. A blood test can show if you have been exposed to the bacteria but cannot diagnose an active infection. Positive results probably mean you have Legionnaires' disease, while negative results may indicate a different infection or insufficient bacteria in the sample. Because Legionnaires' disease can be fatal if not treated promptly, immediate medical attention is important.
Preparation and collection
No special preparation is needed for a urine test. For a sputum culture, you may be asked to drink more water the night before and to avoid eating for 1–2 hours before the test. If a bronchoscopy with bronchoalveolar lavage is planned, you may need to fast for up to 12 hours and stop certain medications, especially antibiotics. Always tell your provider about all medicines you take.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- Blood tests only show past exposure, not current infection.
- A negative result does not rule out Legionnaires' disease because the bacteria may not be present in the sample.
- Prior antibiotic use can affect sputum culture results.
- Other tests (e.g., chest X-ray, Gram stain, acid-fast bacillus, sputum culture, respiratory pathogens panel) are often needed to confirm or rule out the diagnosis.
What to do next
If you have symptoms of Legionnaires' disease (cough, high fever, chills, chest pain, shortness of breath, confusion) and a positive result, your healthcare provider will likely start antibiotic treatment promptly. A negative result may lead to further testing for other causes of pneumonia. Follow your provider's guidance for additional evaluations.