Use of Automated Ambulatory-Care Encounter Records for Detection of Acute Illness Clusters, Including Potential Bioterrorism Events

The advent of domestic bioterrorism has emphasized the need for enhanced detection of clusters of acute illness. We describe a monitoring system operational in eastern Massachusetts, based on diagnoses obtained from electronic records of ambulatory-care encounters. Within 24 hours, ambulatory and telephone encounters recording patients with diagnoses of interest are identified and merged into major syndrome groups. Counts of new episodes of illness, rates calculated from health insurance records, and estimates of the probability of observing at least this number of new episodes are reported for syndrome surveillance. Census tracts with unusually large counts are identified by comparing observed with expected syndrome frequencies. During 1996-1999, weekly counts of new cases of lower respiratory syndrome were highly correlated with weekly hospital admissions. This system complements emergency room-and hospital-based surveillance by adding the capacity to rapidly identify clusters of illness, including potential bioterrorism events. apid identification of unusual clusters of acute illness in the general population is a fundamental challenge for public health surveillance (1). Recent distribution of Bacillus anthracis spores and the resulting occurrence of clinical disease (2) provide new impetus to developing and implementing surveillance systems that can identify both bioterrorism events and naturally occurring illness clusters, such as influenza and waterborne disease. Recognizing individual cases of infection, e.g., inhalational anthrax, requires astute and alert clinicians. However, many potential biological agents of terrorism, including anthrax, have nonspecific prodromal phases, and no explicit diagnosis is ever made for many other syndromes of potential importance. Recognizing these clusters at the earliest possible opportunity will require well-designed surveillance systems to ensure timely detection of unusual clusters of prodromal, nonspecific illness.