Surveillance for the detection of nosocomial infections and the potential for nosocomial outbreaks. II. Development of a laboratory-based system.

We describe the development of a surveillance system that uses thresholds for detecting nosocomial infections and the potential for nosocomial outbreaks based on data from microbiology laboratory records at our hospital from 1980 to 1982. These records were monitored weekly to determine the number of positive isolates by the identity of the organism and by the site of the culture. A mean of 225 specimens was processed weekly, with 60 of these yielding bacteria or fungi. The average number of organisms isolated per positive culture was 1.46. Two methods of establishing thresholds were compared, one based on percentiles of ranked isolates, the other based on the mean plus intervals of standard error. The system using thresholds established by the standard error method was consistently more useful to highlight weeks for which there was high risk of a problem occurring in less time than were the surveillance techniques traditionally employed.