False-positive DNA Probe Test for Legionella Species Associated with a Cluster of Respiratory Illnesses

Between 11 November 1986 and 28 February 1987, legionellosis was diagnosed in 23 patients at one hospital with a recently marketed Legionella-specific DNA probe for respiratory secretions. Only 10 of the 23 probe-positive patients showed findings typical of Legionella pneumonia, including a temperature of greater than or equal to 100.5 degrees F (approximately 38.1 degrees C) and radiographic evidence of pneumonia. No differences were found in the results of laboratory studies, demographic features, or underlying risk factors for these 10 probe-positive patients when compared with the 13 probe-positive patients with nonpneumonic illnesses. A case-control study comparing probe-positive and -negative patients failed to identify any different features of disease or epidemiologic characteristics. Probes of repeat specimens of sputum were still positive 2 to 13 weeks after the initial test in 5 (50%) of the 10 probe-positive patients. The clinical features in most patients were atypical for legionellosis, and the diagnosis could not be confirmed by traditional laboratory tests performed on duplicate specimens processed at the Centers for Disease Control. This report emphasizes the need for clinical microbiology laboratories to confirm test results from new procedures by accepted diagnostic methods.