Pneumonia caused by Legionella pneumophila was first recognized in 1977 after an outbreak among attendees at the 1976 American Legion convention. Previously, diagnosis was usually made by direct culture or by demonstrating a fourfold rise in acute versus convalescent antibody titer to L. pneumophila. However, these methods required an extended interval to obtain the diagnosis and were not helpful at the time of initial presentation. Direct fluorescent antibody testing of lung tissue and respiratory secretions replaced these time-consuming tests. The advent of a urine antigen test provided an even more efficient means of diagnosis. The urine antigen test now accounts for more than two thirds of legionella infection diagnoses. Benin and colleagues reviewed the effectiveness of various diagnostic methods for legionella infection and their impact on the decrease in mortality caused by this strain of pneumonia.
The urine antigen test is highly sensitive and specific for the L. pneumophila serogroup 1 (LP1), which is the causative organism for most legionella infections in the United States. The authors stressed, however, that over-reliance on the urine antigen test would compromise detection and diagnosis of the significant minority of legionella infections caused by other serotypes.
The case-fatality rate for legionella infections has decreased by 66 percent since the introduction of the urine antigen test. The authors postulated that this decrease is likely to be multifactorial. Some improvement may be caused by the wider use of macrolides and quinolones in the initial, empiric treatment of community-acquired pneumonia, which provides adequate coverage for atypical organisms such as Legionella. Earlier diagnosis and appropriate treatment of infection made possible by the easy availability of the urine antigen test also was thought to play a role in the decrease in fatalities. In addition, the authors noted that in the years shortly after the discovery of legionellosis, only the most seriously ill pneumonia patients underwent testing for Legionella, which may have skewed the mortality rates for these earlier cases.
The authors concluded that urine antigen testing for legionella infection, which has become widely adopted over the past decade, may have contributed to improved case-fatality rates; however, they strongly recommend a diagnostic approach that includes both urine antigen testing and culture for legionellae.