What is the role of lab testing in the diagnosis of Legionnaires disease?

Updated: Mar 11, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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While pneumonias caused by numerous pathogens share similar laboratory findings, hyponatremia (sodium < 130 mEq/L) secondary to the syndrome of inappropriate antidiuretic hormone (SIADH) is more common in Legionnaires disease (LD) than in pneumonias secondary to other pathogens; however, this is not specific for LD.

Other nonspecific laboratory findings in LD include the following:

  • Early/mildly elevated liver enzymes

  • Highly elevated erythrocyte sedimentation rate (ESR) (>90 mm/h)

  • Highly elevated ferritin levels (>2 times normal)

  • Increased C-reactive protein (CRP) levels (>30 mg/L)

  • Hypophosphatemia (specific to LD excluding other causes of hypophosphatemia) [13]

  • Microscopic hematuria

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