Despite the potential for high diagnostic precision, biosurveillance researchers and developers group ICD codes into categories ("code sets'') such as "respiratory illness.'' The set of all 60 ICD codes for tuberculosis mentioned above is an example of an ICD code set. It is not necessary to group ICD codes into codes sets, although it is almost always done. For example, we could monitor for the single ICD code for inhalational anthrax (022.1). Creators of code sets usually group codes of diseases and syndromes that share similar early presentations to form syndrome code sets. Respiratory, gastrointestinal, neurological, rash and febrile illnesses are representative of code sets in common use.
A key reason that developers create code sets is to improve the sensitivity of case detection, because patients with the same disease may be assigned different ICD codes. This variability may be due to variability in how coders assign codes or that the patients are at different stages in their diagnostic work-ups. For example, a patient with influenza who has not yet undergone definitive testing may be coded as 780.6 Fever (or any of a number of other ICD codes for symptoms of influenza), 079.99 viral syndrome, 465.9 Acute upper respiratory infection NOS, or 486 Pneumonia, organism unspecified.
The most difficult and "art-more-than-science'' aspect of ICD-code monitoring is development of code sets. The next sections describe several code sets used in biosurveillance. Our purpose is to illustrate how a code set is developed. It is important to note that one code set could be superior (i.e., have better case detection and/or outbreak detection performance) to others for the detection of one disease (e.g., influenza), but inferior to other code sets for the detection of another disease (e.g., bronchiolitis due to respiratory syncy-tial virus). To date, only one study (Tsui et al., 2001) has compared the accuracy of two alternative code sets to determine their differential ability to detect the same set of cases or outbreaks. That study lacks generalizability because no other research groups have found the data they studied—ICD codes for chief complaints assigned by registration clerks—to be available at other institutions.8 Therefore, which code sets are better than others for the detection of various outbreaks such as influenza or cryptosporidiosis remains unknown.
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