Rapid clinical diagnosis of Legionnairesʹ disease during the “herald wave” of the swine influenza (H1N1) pandemic: The Legionnairesʹ disease triad

Background lts hospitalized with atypical community-acquired pneumonia (CAP), Legionnairesʹ disease is not uncommon. Legionnaireʹs 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 Legionnairesʹ 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 Legionnairesʹ 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. ive ingly, 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, Legionnairesʹ disease. During the “herald wave” of the H1N1 pandemic, we noticed an unexplained increase in Legionnairesʹ disease CAPs. Because clinical resources were stressed to the maximum during the pandemic, it was critically important to rapidly identify patients rapidly with Legionnaireʹs disease who did not require influenza precautions or oseltamivir, but who did require anti-Legionella antimicrobial therapy. s on the Winthrop-University Hospital Infectious Disease Divisionʹs diagnostic weighted point score system for Legionnairesʹ disease (modified), key indicators were identified and became the basis for the diagnostic Legionnairesʹ disease triad. The diagnostic Legionnairesʹ disease triad was used to make a clinical diagnosis of Legionnairesʹ disease until the results of Legionella diagnostic tests were reported. The diagnostic Legionnairesʹ disease triad diagnosed Legionnairesʹ 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 (≥2 × n), or hypophosphatemia. The diagnostic Legionnairesʹ disease triad provides clinicians with a rapid way to clinically diagnose Legionnairesʹ disease, pending Legionella test results. s curacy of the diagnostic Legionnairesʹ disease triad was confirmed in our 9 cases of Legionnairesʹ disease by subsequent Legionella diagnostic testing. sions agnostic Legionnairesʹ disease triad is particularly useful in situations where a rapid clinical syndromic diagnosis is needed, ie, during an H1N1 pandemic.