How is it Diagnosed?

About the Disease

Legionnaires' disease is a severe, often lethal, form of pneumonia. It's caused by the bacterium Legionella pneumophila found in both potable and nonpotable water systems. Each year, an estimated 10,000 to 18,000 people are infected with the Legionella bacteria in the United States.

It is not uncommon for patients with Legionnaires' disease to be admitted to the intensive care unit. Some will suffer long-term impaired health-related quality of life.  A study of outbreak survivors showed persistence of fatigue (75%), neurologic symptoms (66%) and neuromuscular symptoms (63%) in months after an outbreak. See Share Your Story for a first-hand account of the severity of this disease.

How is Legionnaires' disease diagnosed?

Specialized laboratory tests are necessary and, unfortunately, may not be available in many hospitals. These include culture on specialized Legionella media. Culture media furnish nutrients for the bacterium. When sputum from the patients is placed onto the culture media, the bacterium grows on the medium and can be identified.


Other tests include direct fluorescent antibody (DFA) in which the bacterium can be stained and becomes visible under a fluorescent microscope. Antibody testing is a blood test in which antibodies that are reactive against Legionella are present in the human body showing that the patient has come into contact with the bacterium previously. Urinary antigen is a test that detects Legionella in the urine.