The genus Legionella includes over 40 different species of fastidious Gram-negative bacilli. While these organisms represent normal environmental flora, many have been shown to cause human disease, most commonly opportunistic pneumonia in immunocompromised patients. The vast majority of such cases are due to L. pneumophila (85%), with the minority most frequently caused by L. micdadei, L. bozemanii, L. dumoffii and L. longbeachae.
For many years the gold standard test for diagnosing Legionella infection has been culture of bronchoalveolar lavage and lung biopsy. A drawback of culture is that results may not be available for several days to longer than a week, since the organism can be quite slow-growing and fastidious. It should also be noted that sputum (expectorated, aspirated or induced) rather than a BAL or lung biopsy is often submitted for Legionella culture and may be less likely to yield positive results.
Rapid testing methods, which include direct histochemical staining of tissue, fluorescent antibody staining of tissue or pulmonary secretions and urinary antigen detection have been useful but frequently lack sensitivity and/or specificity. Urinary antigen detection is specific for L. pneumophila and will not detect the presence of any other Legionella species. Serological diagnosis is highly sensitive, but its utility is generally limited to epidemiological studies, due to the time lag needed to detect seroconversion.
Nucleic acid amplification techniques are attractive tools for detection of Legionella species. Recent studies have shown that Legionella Species by PCR (polymerase chain reaction) offers highly sensitive and specific results for multiple Legionella species with rapid turnaround times of one to two days. Tissue and pleural fluid, as well as respiratory specimens are acceptable for PCR testing for Legionella species. Because of these recent advances in Legionella diagnosis by PCR, MuirLab will no longer offer the culture method.