The use of Legionella testing—especially the Legionella urinary antigen test—is recommended for all pts with community-acquired pneumonia.
- Sputum or bronchoscopy specimens can be subjected to direct fluorescent antibody (DFA) staining and culture.
- - DFA testing is rapid and specific but is less sensitive than culture.
- - Cultures on BCYE medium (with antibiotics to suppress competing flora) require 3-5 days to become positive.
- Serologic confirmation requires comparison of acute- and convalescent-phase samples. Detection of the necessary fourfold rise in titers often requires 12 weeks, but a single titer of 1:256 is presumptive evidence for Legionnaires' disease.
- Urinary antigen testing is rapid, inexpensive, easy to perform, second only to culture in terms of sensitivity, and highly specific. It is useful only for L. pneumophila serogroup 1, which causes 80% of disease cases.
- - Urinary antigen is detectable 3 days after disease onset and generally disappears over 2 months, although positivity can be prolonged if the pt is receiving glucocorticoid therapy.
- - The test is not affected by antibiotic administration.
Treatment: Legionella Infections - Newer macrolides (e.g., azithromycin at 500 mg/d PO, with doubling of the first dose considered; or clarithromycin at 500 mg bid IV or PO) or fluoroquinolones (e.g., levofloxacin at 750 mg/d IV or 500 mg/d PO or moxifloxacin at 400 mg/d PO) are most effective.
- - Rifampin (300-600 mg bid) combined with either class of drug is recommended in severe cases.
- - Tetracyclines (doxycycline at 100 mg bid IV or PO) are alternatives.
- Immunocompetent hosts should receive 10-14 days of therapy, but immunocompromised hosts and pts with advanced disease should receive a 3-week course.
- - A 5- to 10-day course of azithromycin is adequate because of this drug's long half-life.
- - A clinical response usually occurs within 3-5 days after the initiation of parenteral therapy, at which point oral therapy can be substituted.
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