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Diagnostic Testing

Urinary antigen assay and culture of respiratory secretions on selective media are the preferred diagnostic tests for Legionnaires' disease.

Advantages and Disadvantages of Diagnostic Tests

Test Advantages Disadvantages
Culture
  • Clinical & environmental isolates can be compared
  • Detects all species & serogroups
  • 100% specific
  • Technically difficult
  • Slow (>5 days to grow)
  • Sensitivity highly dependent on technical skill
  • May be affected by antibiotic treatment
Urine Antigen
  • 100% specific...
  • Rapid (same day)
  • ...but only for L. pneumophila serogroup 1 (Lp1)
    [ which may account for up to 80% of cases]
  • Does not allow for molecular comparison to environmental isolates
Serology
  • Less affected by antibiotic treatment
  • 80-90% sensitive; 99% specific
  • Must have paired sera
  • 5-10% of population has titer 1:≥256. Single acute phase antibody titers of 1: ≥256 do not discriminate between cases of Legionnaires' disease and other causes of community-acquired pneumonia.
DFA
  • Can be performed on pathologic specimens
  • >95% specific
  • 25-75% sensitive
PCR
  • Rapid
  • Assays vary by laboratory and are not FDA-approved

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