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Legionella Tests

Legionella pneumophila and related species are uncommon causes of pneumonia in both immunocompromised and normal individuals. Legionella is ubiquitous in natural water habitats, and may colonize man-made water features which can then serve as a source of human infections. Cigarette smoking, chronic lung disease, and immunosuppression are risk factors for infection.Tests for legionella infection include respiratory culture, direct fluorescent antibody (DFA) and PCR  for respiratory specimens, serum antibody, and urine antigen detection. 

The gold standard for diagnosis of legionellosis is culture. However, Legionella requires charcoal-containing media for growth, cultures must be held for 14 days, and the reported sensitivity of culture is low at 25-80%. Likewise, Legionella DFA has a low sensitivity because large numbers of organisms are required for visualization. Many hospital laboratories do not offer Legionella culture.  Legionella PCR testing of respiratory specimens is available through a reference laboratory, and is the best alternative when this diagnosis is highly suspected.

Serologic and urine antigen tests are also available, when respiratory specimens are difficult to collect. Serologic testing should include both IgM and IgG antibodies obtained both acutely and during convalescence. Antibody response may not be detectable until one to three months after the onset of illness. The urine antigen test detects a specific soluble antigen present in the urine of patients with Legionella infections and is the test of choice for diagnosis of early infection. It detects only Legionella pneumophila serogroup 1, which causes the majority of cases of legionellosis. The sensitivity is 70% with specificity near 100%. Antigen excretion may begin as early as 3 days after onset of symptoms and persist for up to 1 year afterward. The test remains positive for several weeks following antibiotic therapy.

Specimen requirement for serologic testing is one SST tube of blood.  Specimen requirement for the urine antigen test is 1.0 mL of urine from a random collection. No urine preservatives should be used, and the specimen should be refrigerated after collection.

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