On the threshold.

Due to the October 2001 intentional anthrax releases, the United States has entered an era where the need for rapid detection of outbreaks is understood as never before. Traditionally the nation has relied on astute clinicians and practitioners to identify unusual disease occurrences and notify local or state public health departments in order to initiate an investigation and control measures. Routine surveillance systems, while important for monitoring trends, have been of limited value for rapid detection, as data are neither timely nor complete. The advent of the Information Age brings visions of real-time electronic surveillance systems to complement astute clinicians, but there are many unanswered questions as to how this opportunity can best be realized. In the face of the acute anthrax crisis, many areas instituted enhanced surveillance, requiring hospitals or clinicians to report syndromes, such as influenza-like illness, that may represent the prodromal stages of a potential bioterrorist event due to an agent such as anthrax or plague. Some have advocated the continued utilization of systems requiring manual entry of such syndromic data. However, others have recognized that data entry beyond that occurring in normal clinical workflow is not sustainable in routine clinical settings. In this issue of the American Journal of Preventive Medicine, Lewis et al.1 report on an early detection system that took advantage of the fact that military treatment facilities have common data requirements and assign ICD-9 (International Classification of Diseases, 9th Revision) codes very quickly, unlike the usual civilian approach in which assignment of codes typically occurs later during the process of billing for third-party reimbursement. The Electronic Surveillance System for the Early Notification of Community-based Epidemics (ESSENCE) represents a model for making data available for early detection in near real time. To make such coded electronic data available from the civilian sector will require providers, patients, systems developers, and public health to work together to incorporate standardized data content as well as the capacity to exchange data electronically into clinical information systems—an effort of critical importance for electronic early detection, whether of a natural or intentional outbreak. The ESSENCE system has achieved one major milestone—making electronic data available in near real time. The next questions concern the content and utility of the available data. The ESSENCE paper1 focuses on detection of nonspecific syndromes such as respiratory disease, coma, and diarrhea. As respiratory disease is frequently seen in normal clinical practice, there is a substantial baseline occurrence of the syndrome. For example, Figure 1 in the Lewis et al.1 article demonstrates that an additional 200 cases must be seen in ESSENCE facilities in order to exceed the respiratory disease threshold. Outbreaks large enough to result in over 200 additional cases per day at 26 facilities seem likely to be noted through other modalities. However, as more detailed data from a wider range of clinical sources (e.g., test orders, radiology results, microbiology results, pharmacy data, vital signs) become available in standard electronic format, one can imagine tracking more specific case definitions with less baseline “noise,” such as patients with a pulmonary infiltrate and fever from whom a blood culture was obtained, or laboratory results on rapid antigen testing for pathogens of interest. To address the lack of specificity, syndromic data could also be combined with other nontraditional data sources such as workplace absenteeism or over-the-counter product sales. Rigorous analysis of data collected by systems such as ESSENCE is important for defining the case definitions and analytic algorithms that produce appropriate system sensitivity, timeliness, and predictive value of a positive (defined as detection of an event requiring a public health response). As the systems determine appropriate thresholds resulting in follow-up investigations, they should quantitatively assess the resources required for an investigation when an outbreak is not occurring, as scarce clinical and public health resources are required to investigate false alarms. The Centers for Disease Control and Prevention (CDC) has published criteria for evaluating traditional surveillance systems.2 Since evaluation of rapid detection systems raises additional complex issues, CDC will be working with partners to refine and adapt these criteria (e.g., to incorporate flexibility to alter detection thresholds based on priority for detection and opportunity costs for investigating false alarms). From the Centers for Disease Control and Prevention, Atlanta, Georgia Address correspondence and reprint requests to: Claire V. Broome, MD, Centers for Disease Control and Prevention, Integrated Health Information Systems, 1600 Clifton Road, NE, Mailstop D-68, Atlanta, GA 30333. E-mail: cvb1@cdc.gov.