Rapid clinical diagnosis of Legionnaires' disease during the "herald wave" of the swine influenza (H1N1) pandemic: the Legionnaires' disease triad

Heart Lung. 2010 May-Jun;39(3):249-59. doi: 10.1016/j.hrtlng.2009.10.008.


Background: In adults hospitalized with atypical community-acquired pneumonia (CAP), Legionnaires' disease is not uncommon. Legionnaire's disease can be differentiated from typical CAPs and from other atypical CAPs based on its characteristic pattern of extrapulmonary organ involvement. The first clinically useful diagnostic weighted point score system for the clinical diagnosis of Legionnaires' disease was developed by the Infectious Disease Division at Winthrop-University Hospital in the 1980s. It has proven to be diagnostically accurate and useful for more than two decades, but was time-consuming. Because Legionella spp. diagnostic tests are time-dependent and problematic, a need was perceived for a rapid, simple way to render a clinical, syndromic diagnosis of Legionnaires' disease pending Legionella test results. During the "herald wave" of the swine influenza (H1N1) pandemic in the New York area, our hospital, like others, was inundated with patients who presented to the Emergency Department with influenza-like illnesses (ILIs) for H1N1 testing/evaluation. Most patients with ILIs did not have swine influenza. Hospitalized patients with ILIs who tested positive with rapid influenza diagnostic tests (RIDTs) were placed on influenza precautions and treated with oseltamivir. Unfortunately, approximately 30% of adult patients admitted with an ILI had negative RIDTs. Because the definitive laboratory diagnosis of H1N1 pneumonia by reverse transcription-polymerase chain reaction(RT-PCR), testing was restricted by health departments, resulted in clinical and infection control dilemmas in determining which RIDT-negative patients did, in fact, have H1N1 pneumonia.

Objective: Accordingly, a diagnostic weighted point score system was developed for H1N1 pneumonia patients, based on RT-PCR positivity by the Infectious Disease Division at Winthrop-University Hospital. This diagnostic point score system for hospitalized adults with negative RIDTs was time-consuming. As the pandemic progressed, a simplified diagnostic swine influenza (H1N1) triad was developed for the rapid clinical diagnosis of probable H1N1 pneumonia, which also differentiated it from its mimics as well as from bacterial pneumonia, eg, Legionnaires' disease. During the "herald wave" of the H1N1 pandemic, we noticed an unexplained increase in Legionnaires' disease CAPs. Because clinical resources were stressed to the maximum during the pandemic, it was critically important to rapidly identify patients rapidly with Legionnaire's disease who did not require influenza precautions or oseltamivir, but who did require anti-Legionella antimicrobial therapy.

Methods: Based on the Winthrop-University Hospital Infectious Disease Division's diagnostic weighted point score system for Legionnaires' disease (modified), key indicators were identified and became the basis for the diagnostic Legionnaires' disease triad. The diagnostic Legionnaires' disease triad was used to make a clinical diagnosis of Legionnaires' disease until the results of Legionella diagnostic tests were reported. The diagnostic Legionnaires' disease triad diagnosed Legionnaires' disease in hospitalized adults with CAPs with extrapulmonary findings (atypical CAP) and relative bradycardia, accompanied by any three (ie, a triad) of the following: otherwise unexplained relative lymphopenia, early/mildly elevated serum transaminases (SGOT/SGPT), highly increased ferritin levels (> or =2 x n), or hypophosphatemia. The diagnostic Legionnaires' disease triad provides clinicians with a rapid way to clinically diagnose Legionnaires' disease, pending Legionella test results.

Results: The accuracy of the diagnostic Legionnaires' disease triad was confirmed in our 9 cases of Legionnaires' disease by subsequent Legionella diagnostic testing.

Conclusions: The diagnostic Legionnaires' disease triad is particularly useful in situations where a rapid clinical syndromic diagnosis is needed, ie, during an H1N1 pandemic.

MeSH terms

  • Alanine Transaminase
  • Aspartate Aminotransferases
  • Bradycardia
  • Community-Acquired Infections / diagnosis
  • Diagnosis, Differential
  • Disease Outbreaks*
  • Ferritins
  • Humans
  • Hypophosphatemia
  • Influenza A Virus, H1N1 Subtype*
  • Influenza, Human / diagnosis*
  • Legionnaires' Disease / diagnosis*
  • Lymphopenia
  • Pneumonia, Viral / diagnosis
  • Reagent Kits, Diagnostic
  • Reverse Transcriptase Polymerase Chain Reaction
  • Time Factors


  • Reagent Kits, Diagnostic
  • Ferritins
  • Aspartate Aminotransferases
  • Alanine Transaminase