Legionella Tests
Reviewed
Also called: Legionnaires' disease testing, Legionellosis testing, Legionella pneumophila
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.
Specimen: Urine, sputum, or lung tissue (via bronchoscopy/BAL). Blood for exposure assessment only.
Urgent safety information
Legionnaires' disease can be fatal if not treated promptly with antibiotics. Seek immediate medical care if you have symptoms such as cough, high fever, chills, chest pain, shortness of breath, or confusion, especially after possible exposure to contaminated water sources (e.g., hot tubs, large building plumbing, cruise ships).
Why this test may be ordered
- You have symptoms of Legionnaires' disease (cough, high fever, chills, headache, chest pain, shortness of breath, fatigue, nausea, vomiting, diarrhea, confusion).
- To distinguish Legionnaires' disease from other types of pneumonia, since treatment differs.
- To help confirm the diagnosis in a suspected outbreak or after known exposure to contaminated water.
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.
Urine (clean catch), sputum (coughed or induced), or lung tissue (via bronchoscopy with bronchoalveolar lavage). A blood sample may be drawn to check for previous exposure, but it cannot diagnose an active infection.
Urine and sputum collection have no known risks. Blood draw may cause slight pain or bruising at the needle site, which usually resolves quickly. Bronchoscopy with bronchoalveolar lavage carries very little risk; serious complications are rare but may include bleeding, infection, or pneumothorax (partial lung collapse).
Understanding results
Your healthcare provider will interpret your Legionella test results in the context of your symptoms, medical history, and other test findings. The table below describes the general meaning of the two main result patterns for this test.
- Positive: Legionella bacteria were detected in your sample. This probably means you have Legionnaires' disease. Limitation: A positive result indicates probable Legionnaires' disease, but other conditions may cause similar symptoms. Confirmatory tests are often needed. Next step: Your provider will likely start antibiotic treatment and may order additional tests (e.g., chest X-ray, sputum culture) to confirm the diagnosis and guide care.
- Negative: No Legionella bacteria were detected in your sample. This may mean you have a different type of infection, or that the bacteria were not present in sufficient quantity. Limitation: A negative result does not completely rule out Legionnaires' disease; it may mean the bacteria were not present in the sample or were present in very low numbers. Next step: Your provider may consider other causes of your symptoms or repeat the test with a different specimen type (e.g., sputum or bronchoalveolar lavage).
- 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.
- What do my symptoms suggest about the likelihood of Legionnaires' disease?
- How do these test results compare with other tests I have had?
- If the test is negative but symptoms continue, what should I do next?
- Are there any follow-up tests I should consider?