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Legionella pneumophila causes both community-acquired and nosocomial pneumonia. Cigarette smoking, chronic lung disease, and immunosuppression are risk factors for infection. Tests available to diagnose legionellosis include respiratory culture, direct fluorescent antibody (DFA) detection in respiratory specimens, serum antibody, and urine antigen detection. The gold standard for diagnosis of legionellosis is culture. Legionella requires charcoal-containing media for growth and culture must be specifically requested. The reported sensitivity of culture is 80% with a specificity of 100%. Legionella DFA has a lower sensitivity than culture because large numbers of organisms are required for visualization. DFA sensitivity varies from 33-70% with specificity of 96-99%. The sensitivity of culture and DFA for bronchoscopy specimens is the same as sputum. Because sputum production may be minimal in some patients, serologic and urine antigen tests are available. 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. A single titer of IgG ?1:256 may be compatible with current or past infection. The best evidence of current infection is a change in titer between acute and convalescent sera. Serology sensitivity ranges between 40 and 60% with a specificity of 96 to 99%. 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%. Excretion of Legionella antigen in urine may vary depending on the individual patient and their underlying illness or treatment. Some individuals have excreted antigen for an extended time, so that a history of recent repiratory illness compatible with Legionnaire's Disease should be sought. 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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