Which diagnostic test is performed for Legionnaires disease (Legionella pneumophila) in the evaluation of community-acquired pneumonia (CAP)?

Updated: Oct 31, 2019
  • Author: Stephanie L Baer, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Urinary antigen testing is considered the first-line diagnostic test for L pneumophila. The sensitivity of the test ranges from 55%-99%, with improved sensitivity paralleling disease severity. [49] The specificity is high (95%). The urinary antigen test is applicable only for L pneumophila serogroup type I, which accounts for approximately 85% of Legionella infections in the United States and Europe. In other regions of the world, including Australia, Thailand, and New Zealand, 25% of cases have been attributed to another species, Legionella longbeachae. [59] The urinary antigen test results remain positive for Legionella for long periods but may be negative within the first 48 hours of infection. Using a combination of Legionella UAT and PCR of respiratory secretions, which is often able to detect all species of Legionella, may increase the ability to diagnose Legionella pneumonia. [60] The 2019 IDSA/ATS CAP guidelines recommend obtaining a urine Legionella antigen test in patients with severe CAP and when epidemiologic factors support a potential Legionella diagnosis. [7]

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