Nosocomial legionnaires' disease: lessons from a four-year prospective study.

We studied all cases of nosocomial pneumonia at our 800-bed tertiary care hospital from September 1983 to September 1987. Of the 813 cases of nosocomial pneumonia, 31 (3.8%) were definite (isolation of organism or fourfold rise in titer) and 21 (2.5%) were possible cases (single or stable antibody titer of greater than or equal to 1:256) of legionnaires' disease. The definite cases involved a more severe form of pneumonia and a significantly higher mortality rate--64% versus 14% (p less than 0.0009) compared with the possible cases. Despite attempted comprehensive surveillance, only four (13%) of the definite cases of legionnaires' disease were found that would not have been diagnosed if the study were not ongoing. The yield from adequate (4- to 6-week convalescent serum samples) serologic testing was 5%, whereas the yield from sputum culture was 11%. We conclude that targeted surveillance of immunosuppressed patients with nosocomial pneumonia by culture of respiratory tract secretions for Legionella pneumophila is adequate for monitoring for the presence of legionnaires' disease in a hospital.