How is Legionnaires disease diagnosed?

Updated: Mar 11, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Answer

Culture of respiratory secretions

The definitive method for diagnosing Legionella is isolation of the organism in the respiratory secretions (ie, sputum, lung fluid, pleural fluid). However, Legionella species do not grow on standard microbiologic media but instead require buffered charcoal yeast extract (CYE) agar and cysteine for growth. Optimal growth occurs at 35-37°C.

Legionella is a slow-growing organism and can take 3-5 days to produce visible colonies. The organisms typically have a ground-glass appearance.

Routine sputum cultures have a sensitivity and specificity of 80% and 100%, respectively. Transtracheal aspiration of secretions or bronchoscopy specimen increases the sensitivity. Bronchoalveolar lavage (BAL) fluid provides a higher yield than bronchial wash specimens.

Blood cultures

Legionella can be isolated from blood, but it shows a much lower sensitivity.

Amplification with PCR assay

Polymerase chain reaction (PCR) assay of urine, serum, and bronchiolar lavage fluid is very specific for the detection of legionellae, but the sensitivity is not greater than that of culture. The primary benefit of this procedure, like IFA titers, is that it can be used to detect infections caused by legionellae other than L pneumophila serogroup 1.

Direct fluorescent antibody staining of sputum

Direct fluorescent antibody staining (DFA) is a rapid test that yields results in 2-4 hours, but it has a lower sensitivity and has fallen out of favor. The specificity of DFA is 96-99% using monoclonal antibody instead of polyclonal antibody.

A positive result depends on finding large numbers of organisms in the specimen; therefore, the sensitivity is increased when samples from the lower respiratory tract are used. DFA results rapidly become negative (in 4-6 d).


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