LABORATORY DIAGNOSIS OF LEGIONNAIRES-DISEASE

Abstract

Legionnaires'' disease was diagnosed in 32 patients with nosocomially acquired penumonia by using direct immunofluorescence examination of respiratory tract secretions, indirect immunofluorescence determinations of antibody titer and/or culture of Legionella pneumophila from respiratory tract secretions. Cultures of specimens for L. pneumophila were positive for 13 of 21 patients for whom appropriate specimens were submitted. Using a positive culture as a definition of Legionnaires'' disease, the sensitivity of direct immunofluorescence examination was 62%; that of indirect immunofluorescence serologic studies, 75%. Using any positive test as a definition of Legionnaires'' disease, sensitivity of direct immunofluorescence examination of respiratory tract secretions was 50%, that of indirect immunofluorescence serologic studies (using all 4 serogroups), 80%; that of culture, 65%. A negative control group of 21 patients without Legionnaires'' disease was used to estimate the specificity of the tests. The specificity of direct immunofluorescence examination was 94%; that of indirect immunofluorescence serologic studies, 75%. Culture of specimens was necessary to diagnose some cases of Legionnaires'' disease because the other 2 tests were negative. All 3 tests should be performed for optimal sensitivity and specificity.