A chief complaint is a concise statement in English or other natural language of the symptoms that caused a patient to seek medical care. A triage nurse or registration clerk records a patient's chief complaint at the very beginning of the medical care process (Figure 23.1).

In contrast, an ICD code is a number (e.g., 558.9) that a clinician or professional coder uses to represent a medical diagnosis, syndrome, or symptom-usually for the purpose of billing. The ICD-coding system allows physicians and professional coders to express their diagnostic impression of a patient at different levels of diagnostic precision, ranging from very precise (e.g., ICD code 022.1 for inhalational anthrax) to syndrome (e.g., ICD code 079.99 for viral syndrome) to symptom (e.g., ICD code 780.6 for fever). The diagnosis may be a working diagnosis (a provisional diagnosis) or a definitive diagnosis, although ICD does not allow the clinician or coder to indicate this distinction. A clinician or professional coder may record an ICD code early in the process of medical care. Professional coders, not clinicians, invariably encode hospital discharge diagnoses, which are not available until after a patient is discharged from a hospital. The important points to remember about ICD coding are the heterogeneity in diagnostic precision, who does the encoding, and when the encoding is done.

Chief complaints and ICD codes are used ubiquitously in medical care in the United States in both the civilian and military healthcare systems. Medicare and other third party payers require these data for billing and claims. As a result, the healthcare industry has built significant electronic infrastructure to capture chief complaints and ICD codes.

Over the past six years, researchers have studied methods to obtain and analyze patient chief complaints and ICD codes for the purpose of early detection of outbreaks. The intensity of research on these data has been motivated in part by their availability. The objective of research is to test hypotheses that these data can be used either alone or in conjunction with other data to improve the timeliness of outbreak detection (Wagner et al., 2001). As a result of the research, many health departments are now routinely monitoring chief complaints and ICD codes.

For clarity of exposition, we discuss chief complaints and ICD codes separately in this chapter. However, we do not wish to reinforce a somewhat prevalent impression that they are competing alternatives. Both types of data contain information that is useful in biosurveillance and together they are complementary. In the future, we expect that biosurveillance systems will collect both types of data routinely. They will link these data to other data about a patient to support more accurate inference about a patient's true disease state. We explore the future roles of chief complaints and ICD codes and their synergies in the final section of this chapter.

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