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Comparison of Outpatient Visit and Emergency Department Data for Use in Syndromic Surveillance --- New York City, 20012004

Kristina Metzger, F. Mostashari, M. Kendall
New York City Department of Health and Mental Hygiene, New York City, New York

Corresponding author: Farzad Mostashari, New York City Department of Health and Mental Hygiene, 125 Worth St., Box 6, New York, NY 10013. Telephone: 212-788-5384; Fax: 212-788-4473; Email:

Disclosure of relationship: The contributors of this report have disclosed that they have no financial interest, relationship, affiliation, or other association with any organization that might represent a conflict of interest. In addition, this report does not contain any discussion of unlabeled use of commercial products or products for investigational use.


Introduction: Monitoring outpatient visits might enable more timely and sensitive syndromic surveillance than emergency department visits because of higher daily volumes and the potential for capturing illness at an earlier stage.

Objectives: Data from all 11 public hospitals in New York City with outpatient and emergency departments (EDs) were evaluated to compare the usefulness of these two data sources for monitoring communitywide respiratory and gastrointestinal illness.

Methods: Historic data were obtained on outpatient and ED visits during November 1, 2001--May 31, 2004. Demographic characteristics of patients were compared. The seasonal and temporal trends of comparable syndrome categories (respiratory, fever/viral, asthma, and gastrointestinal) were examined and coded according to the International Classification of Diseases, Ninth Revision (ICD-9), diagnosis code for outpatient visits and chief complaint for ED visits. For each syndrome, timing and frequency were assessed for 1-, 2-, and 3-day temporal clusters with a 14-day baseline period by using temporal scan statistics with SaTScan™ software.

Results: On weekdays, more patients visited outpatient clinics (mean: 3,727) than EDs (mean: 2,906). On weekends, limited outpatient visits occurred (mean: 95); EDs had a slightly lower volume than on weekdays (mean: 2,492). Compared with the ED population, the outpatient population included more patients aged <12 years and >65 years, more females, and more minority patients and those on Medicaid. The temporal trends of the respiratory syndrome from outpatient clinics and EDs were strongly correlated (r = 0.67), as were the fever/viral (r = 0.57) and asthma (r = 0.60) syndromes, but less correlated for the gastrointestinal syndrome (r = 0.36). Citywide temporal clusters were occasionally detected on the same day.

Conclusion: This evaluation of outpatient visits indicates that this data source might be potentially useful for syndromic surveillance, particularly in conjunction with ED data. The demographic characteristics differ, allowing for the examination of complementary populations, and might explain certain differences in observed temporal clusters. Whereas few outpatient visits occur on weekends, both data sources have comparable overall daily volumes. These data sources could be examined simultaneously by using multivariate methods, which might increase the power to detect outbreaks. Outpatient visit data include clinician and patient information as well as diagnoses, which substantially increases the feasibility of cluster investigations. Using both outpatient and ED visit data as part of syndromic surveillance might enhance the ability to detect and validate outbreaks.

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Date last reviewed: 8/5/2005


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