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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

TestAdvantagesDisadvantages
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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