Recommended Testing for Legionnaires' Disease Misses Cases

The guidelines concerning testing patients for possible community-acquired pneumonia due to Legionella may underestimate the number of cases being seen by clinicians.

These guidelines recommend testing for the urine antigen of Legionella in patients with any of the following: severe pneumonia requiring intensive care unit admission, failure of outpatient antibiotics, active alcohol abuse, history of travel within previous two weeks, or pleural effusion.

Doctors at the Rhode Island Hospital (Providence, RI, USA) carried out a retrospective study that identified nearly 4,000 patients with a primary or secondary diagnosis of pneumonia in an 18-month period. Of these, 1,406 patients (35%) had a Legionella urine antigen test and/or Legionella culture as follows: 1,251 had a urine antigen test alone; 49 had a Legionella culture alone; and 106 had both a Legionella culture and urine antigen test. During the same time period, 626 patients underwent bronchoscopy and had bronchoscopic specimens sent to the microbiology laboratory for culture, 277 (44%) of whom had Legionella testing done as follows: 122 had a Legionella culture alone; 31 had a urine antigen test alone; and 124 had both a Legionella culture and urine antigen test.

The study found that if testing was only done in patients felt to be at increased risk of Legionnaires' disease based on the current guidelines, more than 40% of Legionella cases could be missed based on this single-center study. Nearly all of the cases were community-acquired and all cases were L. pneumophila serogroup 1. Thirty-six of the 37 cases were diagnosed solely by urine antigen testing. This suggests that cases of Legionnaires' disease due to other serogroups may have been missed because Legionella culture was underutilized. The scientists suggest more widespread testing for Legionella in patients admitted to hospitals with pneumonia.

Leonard Mermel, DO, medical director of the epidemiology and infection control department at Rhode Island Hospital, said," This single-center study suggests that current recommendations for Legionella testing will result in missed cases. More widespread testing will identify additional cases allowing focused antimicrobial therapy and will alert public health officials of such Legionella cases.” The study was published on September 7, 2011, in the Biomedcentral's online journal BMC Infectious Diseases.

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